ARISTOTELIAN FALLING BODIES. [DES] Aristotle taught that the “natural motion” of a falling
bodies is to fall equal distances in equal times. He also said that the speed
of fall is proportional to the mass. Is this correct? Can we test it? Bodies
falling in fluid media actually fall with constant speed once they reach
terminal speed. So, at terminal speed a body does fall equal distances in
equal times. This can be demonstrated with ordinary coffee filters from coffee
machines, the paper cups sold for baking muffins, or the small cups used
for candies or party favors. When dropped with closed end down, their motion
is rather stable, and they reach terminal speed very quickly.
But what about the relation of speed to mass? This may be studied quantitatively.
The cups may be nested, presenting nearly the same aerodynamical profile
for one, two or more cups. Drop one cup and a stack of two simultaneously,
from different heights. How much higher must you drop the stack so that it
lands simultaneously with the single cup? How about a stack of three? Someone
may have to stand on a ladder to drop the heavier stack. (Alternatively,
both may be dropped simultaneously from the same level, one landing on the
desk top, the other landing on the floor. Adjust starting level till they
land simultaneously.) In our experiments a stack of four must be dropped
from twice the height as a single one to land simultaneously. What does this
tell you about the relation between speed and mass? In these times of shrinking
equipment budgets a quantitative experiment such as this, which uses only
inexpensive materials, is most welcome. [Answer]
We cannot perceive acceleration of a falling body by eye. A classic way to
convince students that falling bodies accelerate uses wooden balls, metal washers,
or large metal nuts affixed to a
long cord that is hung from the ceiling. The balls are spaced apart so that, from
bottom to top, they get farther apart, just the right amount so that when the upper end is released the sound of their impacts hitting the floor is a series of clicks equally spaced in time. Start with balls at zero, 30 cm, 130 cm, and 2.74 m from the floor.
The bottom ball should always be initially touching the floor when the system is released.
This demo isn’t often done because of the nuisance of the setup and hanging and releasing the cord from the ceiling. A more practical version uses Galileo’s “diluted gravity” method, rolling a ball down a grooved track with a slight inclination. Obtain a 6 foot long extruded aluminum V-strip from the hardware store. Make a suitable block to incline one end a couple of inches above the lecture table. A metal ball will roll down the trough slowly. Students probably won’t “see” the acceleration. But if you place small paper clips along one edge of the ramp, the metal ball will make a click as it encounters each clip. Now space the clips for equal clicks. It will now be apparent that the clips increase their separation down the track. Always start by releasing the metal ball from the same position and mark that position for reference. When you have perfected this, you may want to calculate the clip positions, and replace each clip with a small groove filed in the aluminum. These will be permanent, and will produce audible clicks. Galileo would have loved it. The apparatus is easily stored in the stockroom. [DES]
BALL IN FUNNEL. Blow into the small end of a funnel containing a ping
pong ball. The ball will not fall out even though the funnel is inverted,
so long as air is moving between the ball and the funnel wall. [HG]
Stores that sold vacuum cleaners used to attract customer’s attension by setting up this demo with a vacuum cleaner blowing air through a funnel, and a lightweight colorful ball bouncing above the funnel, suspended there. Would this also work if the vacuum cleaner were reversed, to “suck” air through the funnel? Are you sure that isn’t the way they were doing it?
Two balloons, partly inflated, are connected at either end of a tube with
a stopcock. (A pinched-off section of rubber tubing may be used.) One balloon
is inflated more than the other. When air is allowed to pass through the
connecting tube, what happens?
- The large one deflates completely.
- The small one deflates completely.
- The large one deflates and the small one inflates until they attain the same
This demonstration doesn’t work reliably with balloons. The outcome depends
upon the degree of inflation of the two balloons. One could consistently
use the same brand of balloons, and practice in advance to get the correct
initial inflation, but isn’t that something of a fraud? The demo is much
more reliable with soap bubbles, but more trouble to do.
The only excuse for doing this demonstration is to illustrate a detailed
calculation of the dependence of surface tension upon pressure. This is a
bit too much math (vector free-body diagrams in three dimensions) for most
high school courses.
If you do
it, and do it successfully, there’s a neat joke you can include. Do this
demo with the largest balloon below the other, and say that gravity pulls
air down into it to make it larger.
Question: Why do balloons rise in the atmosphere? Answer: Because
someone let go of the string.
Question: Does an inflated balloon weigh more, less, or the same as
it does when deflated? By “weigh” we mean what it would weigh when placed
upon a sensitive balance scale. Answer: More. The air inside is
compressed. This may be easily tested with an electronic balance.
Showmanship tip: Inflate the balloon, tie it off, then fasten the
neck of it to the pan of the balance. Note the balance reading. Puncture
the balloon and note the change in the balance reading. Better yet, use a
sensitive simple mechanical balance, so that the audience can see
the scale unbalance when the balloon is punctured.
Serious question: When is a balloon hardest to inflate? Answer:
When it is smallest. Actually the pressure barely reaches 1.06 atm in the
early stages of inflation, then drops to about 1.05 atm and changes very
little from then to full inflation. That’s why the two-balloon demo is so
unpredictable. Sometimes the two different size balloons just sit there,
without either one deflating.
BALLOON ELECTROSCOPE. Light rubber balloons suspended from long silk strings
act as demonstration electroscope. Charge the balloons by rubbing them with
woolen cloth or fur. When charged, the balloons can be made to stick to flat
surfaces such as walls or ceilings. [HG] With young children, consider adhering
the charged balloons to the clothing of a volunteer from the audience.
BERNOULLI ATTRACTION. [MG-1] Lay two books on the table with their spines
near each other and parallel, forming a channel on which two ping pong balls
can roll. Place the balls a short distance apart and blow horizontally in
the space between them. They roll toward each other.
BERNOULLI BALANCING ACT. Bernoulli’s principle can be shown by balancing
an inflated balloon or beach ball on a jet of air from the output end of
a vacuum cleaner. The balloon will hover near the ceiling and will not fall
off even when the air jet is tipped at a considerable angle. A ping pong
ball balanced on a fine jet of water illustrates the same principle. [HG]
You can even lie on your back on the floor and blow upward to support a ping-pong
ball. Or blow through a vertical tube. Or stand up and blow through a tube
bent in a right angle, with the open end up. [DES]
BIG TORQUE. Hold the end of a broom handle in one hand and extend your arm
and the broom handle horizontally in front of you. Tie a string around a
book and hang the book under the stick a few centimeters from your hand.
Try to keep the stick horizontal while someone slides the book toward the
end of the stick. Although the weight of the stick and book do not change,
the torque increases. Lever arm has real meaning here.[HG]
BOBBING ON THE WAVES? [BERG] A beaker of water is suspended from a spring. An object
floats in the water. As the beaker-pendulum moves up and down, what happens
to the floating object as the pendulum is at the bottom of its motion. Does
it rise up out of the water, sink deeper, or stay at the same level relative
to the water surface?
Result: It stays at the same level relative to the surface. Even when
you whack the beaker from the bottom the float stays at the same level. Of
course a detailed analysis with a free body diagram is in order here, or
nothing will be learned from the demonstration.
BLACK BOX CONTENTS. Into a small box, place small objects and seal the box
closed. Students can manipulate the box, and try to tell you: (a) How
many pieces are in the box. (b) the shape of the pieces. (c) How heavy the
pieces are (density). (d) How big the pieces are. (e) The color of the pieces. By
doing this the student has reason to believe that scientists may know something
about the atom even though it has never been seen; just as the student knows
something about unseen objects in the box. [HG]
I recommend boxes with the following: (a) one wooden sphere, (b) one wooden or plastic cube (from a pair of dice), (c) one wooden rod or cylininder, (d) a small amount of sand, (e) some water in a closed bottle, (d) some with two or more objects, (e) a wooden ball attached with a cord to the center of one cube face, (f) a wooden ball attached with a cord to the inside corner. In the last two, the cord should be shorter than the cube edge, so that it can only touch the nearest faces of the cube. A chemistry colleague also used to make some cubes with inner compartments, splitting the cube into two equal chambers with a wall down the middle. [DES]
BOILING WITH ICE. Fill a flask two-thirds full of water and bring it to a
boil. Cork the flask and invert it, taking care that the hot water does not
spill out. Place an ice cube on bottom of flask. As the ice melts, the water
begins to boil again. If the flask is corked with a one-hole stopper with
a glass tube extended almost to the bottom of the flask, boiling can be effected
by reducing the pressure of the entrapped air. You can also run cold water
over the flask. Have the student feel the flask temperature as the ice boils
the water. [HG]
If a slice of buttered toast is accidentally brushed off a tabletop, does it usually land buttered side down? Yes, but why? Buttered toast is messy, but this experiment works just as well with a wooden slab the size of a slice of bread, painted yellow on one side (simulating butter). Or any flat object with one side marked.
See also coindrop and dropping things.
BUTTON-AND-STRING SPINNER. Children used to make these as toys, using a large
coat-button from their mother’s sewing basket. Loop a string or heavy thread
through two opposing holes of the button, and tie the ends together. Put
the loop over your thumbs with the button hanging. Spin the button around
until the strings are well-twisted on both sides. Then pull the strings taut.
The button will spin rapidly as the string unwinds. When nearly unwound,
release the tension, but keep the string relatively straight between the thumbs.
The angular momentum of the button will cause the strings to twist again
in the opposite direction. As the button slows, pull on the string again
and its spin direction will reverse.
This is more interesting to do than to watch. One learns to modulate the
tension during the spin for maximum performance. The tactile “feel” of this
process is fun to experience. If the button has slits or a raised repetitive
design it may make a musical “whirr” as it spins, the pitch being speed
dependent. Lots of physics here! You could even punch or drill equally spaced
holes just inside the rim, to produce clear tones.
This demonstrates conversion of energy from kinetic (spinning rapidly)
to potential (string “wound up”). There’s considerable friction loss, so
one must continually supply energy by pulling on the string at the most advantageous
times. Compare this to the person pushing a child in a swing. The push must be
timed to be just after the maximum amplitude is reached. If you push just
before that you’ll take energy away from the swinging system.
The string should be quite pliable, and not too heavy. In fact the optimum
weight/length of the string is dependent on the weight of the spinner. The
holes in the button should not be too close together.
Other spinners to consider: The plastic or metal gears or pierced disks from
Erector (TM) or Meccano (TM) sets work well. Colorful spinners may be constructed
from K-Nex (TM)construction sets. [DES]
Color mixing: Consider pasting colored sectors to one side of a
larger size spinner, using the three primary colors. When spun, you may get
white. The angular size of each sector can be adjusted until the white is
quite pure (the three color sectors won’t be equal in angle). Mixing just
two sectors can give you green from yellow and blue, orange from red and
yellow and purple or magenta from blue and red. This is additive color mixing.
(Purple and magenta are not spectral colors, but are achieved only by mixing.)
Persistence of vision: A different version of button-string-spinner
can be used. This uses a disk with four holes, in pairs of two on opposite
sides.Two strings connect to these holes, so that the card may be spun, not
in its own plane, but around its diameter. Experiment with hole spacing,
card weight, and string weight for best performance. Now two pictures are
put on opposite sides of the disk, which will be “mixed” when the disk is
spun. A common illusion is a picture of a bird cage on one side and a bird
on the other. When spun, the bird appears to be in the cage. Mixing of two
colors may also be demonstrated. [DES]
A wooden slat, or yardstick is suspended by two thread loops, then struck
hard at its center. The wood will break without breaking the strings. The
wood may be suspended from loops of paper held up on razor edges. Pins may
be stuck in the ends of the wood, and the pins supported on the rims of two
wine glasses. Once you convince yourself that very little impulse reaches
the supports before the stick halves drop, you realize that the stick may
be suspended from loops around the ears of two students, without any “Vincent
van Gogh” outcomes. Carnival performers used to do this with the stick suspended
from their eyelids.
CHAIN REACTION. Arrange wooden matches closely on a soft board by means of
straight pins placed through them at their midpoint. Hold the board upright
and ignite the bottom match. The others will follow in turn to simulate
a chain reaction. The usual warnings about “playing with
fire” apply here. [HG]
Push pins through a thin board in a regular triangular array about 1 cm apart.
The board is in a vertical plane. Stick the back end of matches onto the
pins. Light a match near the bottom of the board. Chances are it will ignite
the two nearest matches above, and so on, demonstrating the multiplication
factor of a chain reaction. I have not tried this, but it could conceivably
work with the board in a horizontal plane if the matches are closely enough
spaced. This would be more realistic, in that the reaction would spread in
all directions from the initial match. But to make it visible by an audience,
a large mirror would have to be placed above, at a 45° angle.
If this is feasible, one might consider dispensing with the pins and using
a board of metal or other fireproof material with an array of holes drilled
in it to anchor wooden matches. The pins in the above demos were only to
protect the wooden board from being scorched. [DES]
Of course the best chain reaction simulation is still the large array of mousetraps,
cocked, with two corks resting on each one. The entire array is in a chicken-wire
enclosing box. Triggering one trap releases two corks (analogous to neutrons)
that bounce within the box and may trigger two other traps, which release
two more corks, and so on. When done properly, it seems that the whole thing “goes
off” in an instant, ending up in a jumble of mousetraps and corks.
Old brick factory chimneys are demolished by explosive charges at their base.
If tall enough the chimneys do not topple as a single unit, but break in
two parts as they fall, at a point about halfway up. This may be easily
demonstrated with two wooden cylinders on a tiltable board, as shown. Children
demonstrate it with stacks of wooden blocks, doing physics at a very early
age. But they don’t try to explain why it happens. Can you? [DES]
Brick structures have very little resistance to being pulled apart at the
mortar lines. Poured-concrete chimneys, especially if they have steel reinforcing
bars, would behave differently under these conditions of demolition.
Show that this doesn’t happen with a stack of two cubes. What does happen?
Now try higher stacks of cubes or cylinders. How high must the stack be before
you see the definite “breaking point” occur partway up the stack? What
conclusions can you draw? Does the area of the base relative to height play
a role? Does the cross-section of the stack at the point of the break play
Hold two quarters and a nickel as shown, between thumb and forefinger. Release
pressure slowly, allowing the bottom two coins to drop. The quarter will
land on top of the nickel if the distance of drop is correct. Practice to
find that distance. Why do the two coins usually turn over as they fall? See also buttered toast.
COLOR ABSORPTION. Using colored pencils, draw a bird in a blue cage. Let
the bird out of the cage by covering the drawing with a red filter. Try other
color combinations to show the effects of color absorption by filters. [HG]
COLORED COCKROACHES. Many organic substances, dead or alive, show interesting
characteristics under ultraviolet (UV) light. Cockroaches are multicolored under
ultra-violet light. [HG] The “whitening” agents in laundry products cause
white shirts to glow in UV. Some postage stamps have hidden printing visible only in UV. Of course many rock and mineral specimens glow in various colors in UV.
CONDENSATION OF WATER VAPOR. A simple cloud chamber can be made from a gallon
jug fitted with a one-hole stopper with a short piece of glass tubing. Blow
into the jug through the glass tubing to increase the pressure. Put a finger
over end of tube and pull out the stopper, suddenly reducing the pressure.
No cloud is formed. If some smoke is introduced into the jug it provides
nuclei about which water vapor condenses. Repeat the performance and watch
the clouds form in the jug. [HG]
COMING AND GOING. Weld two semicircular steel rods together and weld or fasten a heavy
ball at each end. Paint the balls different colors, say red and green. At
the center you need something like a ball bearing to allow it to rotate with very small friction with respect to the flat plate that will be placed on your head. When wearing it you’ll look like a space alien (see picture). Then when you turn and walk.
The balls keep the same orientation in space. This comes close to being a
pure demonstration of static inertia. [Many demonstrations that claim to demonstrate inertia actually are not so clear-cut, having complicating features.]
Presentation: “This is great for someone who never knows whether he’s
coming or going.” Walk with the red ball in front of you. “If the red ball
is in front of your face you are going.” Turn around and walk the other way.
“And if the blue ball is in front of you, you are coming.” [Dick and Rae]
COTTRELL PRECIPITATOR. Attach one lead from a spark coil to a foil surrounding
a glass tube of about one-inch diameter. Extend a wire from the other terminal
of the spark coil through the tube, insulated from the foil. Place a small
amount of hydrochloric acid in one flask and some ammonium hydroxide in a
second flask. With glass tubing connect the flasks and large glass tubing
in train. Blow air into the first flask causing ammonium chloride to be forced
into the Cottrell precipitator. Activate the spark coil and see ‘smoke’ consumed.
It works near instantly on cigarette smoke. [HG]
CRUSHED CAN. The force of normal external air pressure is sufficient to collapse
a rectangular varnish can. In a clean can place a few tablespoons of water
and bring it to a boil to expel the air with the water vapor. Close the cap
tightly as soon as water boils vigorously. Cool the can by dashing cold water
on it. Two comments: You don’t need to run cold water over the can. Be sure
to stopper the can very soon after you remove the source of heat. If you
don’t, the reverse effect happens. [HG] I’ve seen this done with 50 gallon steel drums. This quickly turns a useful container into scrap metal.
DENSITY OF ICE. Some properties of water make interesting conversation pieces.
Demonstrate that ice is lighter than water by placing a large icicle in a milk
bottle (ice cubes may be used). Add cold water to fill jar while holding the
ice under the water. Let the ice float and observe how much water overflows as
the ice melts. [HG]
DIMPLES AND PIMPLES.
[Use safety glasses and protect nearby
students with a shatterproof glass or plastic shield between them and the
experiment.] Heat a spot on a cold light bulb with a blow torch and
a dimple will form in the glass. Light the bulb and again heat a spot until
a pimple forms. [HG] The fact that the dimple is indented shows that there’s lower pressure inside the bulb. If a bulb is broken, it doesn’t explode, it implodes.
DISAPPEARING GLASS. Some liquids have an index of refraction very close to
that of glass. When a glass object is lowered into the liquid, it almost
disappears. For some materials the dispersion of the liquid doesn’t match that
of the glass, so a faint rainbow image of the object still can be seen.
For safety and good effect, the best combination I’ve found is Pyrex (TM)
glass in Wesson canola oil. The oil is nearly clear; just slightly yellowish.
Small Pyrex beakers immersed in it show only a faint outline, but, of course,
the frosted label remains visible, like the grin of the Chesshire Cat. A
diligent search of the lab stockroom may turn up some pyrex glassware without
Clean the glass thoroughly to remove any surface film.
For the adventurous who’d like to use other materials, here’s some information
from various sources. Some readily available liquids already are near-matches
for the refractive index of crown glass, which is 1.5170 for the sodium D
- Borosilicate crown glass, n = 1.5170
- Spectacle crown glass, n = 1.5230
- Fused quartz, n = 1.4585
- Pyrex glass, n = 1.48
- Fluorite, n = 1.4338
Benzene (Benzol) and Carbon tetrachloride (1:4). This is the best for use
with crown glass, but both ingredients are carcinogenic.
- Anisole (expensive), n = 1.5179
- Toluene (1.4961) and Methanol (1.3288). I don’t know the ideal ratio of volumes.
- Glycerine, n = 1.47
- Cedar oil.
- Johnson’s Baby Oil.
- Castor oil, n = 1.48
- Olive oil. Unfortunately it’s yellow.
- Corn oil, or other vegetable cooking oils.
- Xylene, n = 1.5055
- m-Xylene, n = 1.4972
- p-Xylene, n = 1.4958
Also consider these combinations:
- Quartz (1.46) in glycerine (1.47).
- Lucite (1.47) in glycerine (1.47).
Some mixtures may be concocted by trial and error. Be careful,
because some of the ingredients are carcinogenic. If you use those, keep
them in sealed bottles and avoid skin contact or breathing the vapors.
The index of the mixture is not simply the average of the indices
of the components.
Lower a glass stirring rod into the liquid. The portion below the surface
disappears. Lower the glass end of an eye-dropper (crown glass) into a liquid
of the same refractive index. The glass disappears, but the image of the
air inside punches into the liquid. Draw up some liquid. Remove the dropper.
Squeeze out drops, and it seems if they are drops of glass from the end of
Keep a small beaker in a stoppered bottle of the liquid. The beaker vanishes, but
its frosted label won’t. Tilt the bottle so the beaker is partly exposed
to reveal that it’s really there.
Let a coin rest on a small submerged transparent solid object. The coin seems
to be suspended in the liquid.
DROPPING THINGS. Here’s a simple scenario to convince students of the importance
of words and of specifying things completely and precisely. Hold up a book
and a sheet of paper. Drop them simultaneously from the same height. Note
the behavior of the paper. Try dropping them with the paper dropped edge
down. The book wins each time.
Hold the book in one hand, paper in the other, hands wide apart. “I claim
that I am clever and skillful enough that I can drop both of these from the
same height simultaneously, yet the paper will reach the floor first.” Students
are probably doubtful of this claim. Place the paper underneath the book
and drop it. “Not fair!” say the students.
“OK, this time I promise not to put the paper under the book. Yet I claim
that when the book hits the floor, the paper will be within 1 cm from reaching
the table.” This time you put the paper in the book, between the back cover
and the pages.
“I can do better. I will place the paper on TOP of the book, then drop them together and the paper will reach the table first.” Hold one finger under one end of the book, and release the other end so that the book rotates as it falls. Do this from a height of about 32 inches above the table. Experiment to get the ideal height so the book makes exactly 1/2 revolution. (See buttered toast.)
Finally: “This time I promise to drop them simultaneously from the same height,
one in each hand, so the paper never touches the book.” After they agree
that this sounds fair, crumple the paper and wad it into a tight ball before
dropping. They land simultaneously.
DRY WATER. Let the student propose explanations why one can pick a coin from
the bottom of a beaker of water that has been dusted with Lycopodium powder
and not wet a finger. The beaker was filled with water and the powder dusted
over the water surface. [HG] Lycopodium powder is the pollen of the Lycopodium
fern. Frankly I don’t have an explanation off the top of my head, and haven’t
encountered one in the literature. [DES]
EGG PHYSICS Here’s some eggcellent demos with eggs, which we used to do when
we were children, and so did our parents and grandparents. Some of these
appeared in the Chicago Sunday Times, March 24, 1940, collected and presented
by Martin Gardner.
Eggs, fresh or old, sink in cold water, at least in my experience. But they
are so near the density of water that dissolving sufficient salt (several
teaspoonsful to a glass) in the water will make them float. In fact, you
can get a tall glass cylinder, put some salt water in it and float the egg
on the salt water, then carefully add plain water so it doesn’t mix with
the salt water, and the egg seems to float halfway between the surface and
the bottom. I’m not talking about cooking the egg here—that’s another
As-yet untested proposal.
Since an egg is so close to the density
of water, how about using it in an oversize cartesian diver, visible from
the back row! You might have to salt the water a bit to make it barely float.
Cap the container, and squeeze its sides.
An egg in boiling water often rises to the surface because of convection
currents in the water. But why does it stay at the top? For the same reason
that large pebbles in a container of small ones rise to the top and stay
there when the container is shaken. The pebbles develop convection currents
Balance an egg on end. You can do this at the dinner table on a tablecloth,
or on a smooth table surface. Surreptitiously make a small pile of salt.
Balance the egg on it, then blow the loose grains away. Only the grains wedged
under the egg (which are the only ones supporting it) will remain, and not
Second method. Shake the egg forcefully to break the yolk. Then the egg can
be balanced on its broad end because the heavier globs of yolk sink to the
bottom, lowering the center of gravity. This can prompt a discussion of the
center of gravity relative to the center of curvature and the conditions
To balance a hard-boiled egg on end, gently crush the shell at the end and
sit it there.
To tell a boiled egg from a fresh one, spin it on its side. The hardboiled
eggs spin longer. The fresh ones lose energy from viscous drag. But a fresh
one will spin longer when spun on one end around its long axis than it will
when spun on its side. Why?
Spin a fresh egg on the table. Stop its rotation momentarily with your finger,
then quickly release it. It resumes spinning for a bit. When you stop the
shell abruptly, the yolk and white continue to rotate. If you release the
shell before that rotation stops, viscous drag pulls the shell along with
the yolk and white.
Most people cannot break an egg by clasping it between their palms, with
the two ends touching the palms. Try this with one of the football
players or wrestlers in your class. If they fail, they will challenge you
to do it. And you can. How?
Coat an egg with soot from a burning candle. Immersed in a glass of water
it looks silver.
A hardboiled, peeled, egg can be forced into a milk bottle by the usual physics
trickery. Burn a wad of paper in the bottle. When the flame is established
strongly, quickly place the egg in the bottle’s mouth. It helps to moisten
the egg first. As the air inside is heated it expands, forcing air out around
the egg. Then as the air cools inside, the pressure drops inside the bottle,
and the egg is pushed inside by the greater air pressure outside. The egg
can be removed by turning the bottle upside down so the egg blocks the neck.
Then close your lips around the bottle’s mouth and blow into it, creating
enough pressure to force the egg out. It pops out surprisingly quickly, so
be ready to catch it. Note: Avoid any explanation you may have seen
in books that talks about the flame “using up oxygen” in the bottle, for such
processes have negligible effect on the pressure in the bottle.
Another as-yet untested proposal. How about an unpeeled egg in a bottle?
Soak an uncooked egg in strong vinegar until its shell softens. Then the
egg may be sucked into the bottle by the method previously described. Now
flood the egg with cold water till the shell hardens. Pour out the water
and let everything dry. The egg in the bottle now seems impossible. I have
not tried hardboiling the egg first, or boiling it after it is in the bottle.
Wrap a string around the egg. Try to burn the string with match or candle
flame. The string won’t burn because the thermal energy of the flame is absorbed
by the shell and the string can’t reach kindling temperature.
An egg can be tossed into the air and caught on a plate without breaking.
It takes practice. Martin Gardner says that performer Bill Talent used this
in his juggling act. He’d put an egg on the floor between his heels, jump
with both feet, propelling egg into the air behind his back, then catch it
on a plate held in one hand.
Of course, eggs are used in the so-called inertia demos. Fill four glasses
with water. Put them under the four corners of a flat tray with raised edges.
Roll up four playing-cards and secure them with rubber bands. Use them as
supports for four eggs positioned directly over each glass. Knock out the
tray with a sharp blow of the palm. Eggs will fall into the glasses. Water
can be put in the glasses if you don’t want the eggs scrambled, but the water
will splash out on the table. The smashed eggs won’t. (You hope!)
Write a message on the eggshell with a solution of one ounce alum to one
pint vinegar. The writing is invisible, but penetrates the shell. Boil the
egg. The writing still isn’t visible, but when the egg is peeled, the message
is seen on the solid egg white.
No matter how hard you throw a raw egg into a curtain or suspended bed sheet
the egg will not crack open. Because the sheet “gives” as the egg makes contact,
the deceleration time for the egg to change its velocity to zero is fairly
long. Therefore the sheet applies a large stopping impulse (force x time)
with a very small force and a long time. However, if the uncracked raw egg
falls out of the sheet and hits the floor, it comes to a quick stop with
a large force and… An omelet or scrambled eggs anyone? [HG]
This one is more of a trick than a demo. One must be careful about doing
tricks in a class setting. This one can be used in this manner: “Here’s a
demo that seems to defy physics. But of course nothing violates correct
physics. Sure it’s a trick, but just like magician’s tricks, there’s a physical
explanation. So what is a physical mechanism that would make this happen?
What’s your model of how it’s done?”
First the preparation. Blow an egg. You all know how to do that, don’t you?
If not, here’s the method. Shake it to scramble the contents. Put a needle
hole in each end and blow into one of these to force all the contents out
the other end so you have an empty shell. Leave it a few days to let it dry
inside. Now that you know how to do it, blow another egg, for use later.
Actually, for these demos, I think it better not to make the holes
in the ends of the egg, but elsewhere.
Close one hole in the egg with wax or white glue. Enlarge the other hole
if necessary to allow fine sand to be put inside the shell. Close the hole
with wax or white glue. This “egg” will now balance on either end or in any
other cockeyed position. Use your ingenuity and balance it precariously on
things. This is the “obedient” egg, for it balances any way you want it to.
The other egg you have prepared similarly. It is to be the disobedient egg,
that will balance only one way. Decide, before you blow it, how you want
it to balance. Perhaps on the small end? Then be sure to make the needle
holes well-removed from the point of intended balance. This time mix finely
shaved candle wax with the sand and put some inside the egg. Close both holes
with glue. Let dry. Heat the egg, point down, till the wax melts and the
wax-sand glob is adhering to the inside of the egg-shell. Let it cool, and
it balances on its tip. Even worse, if you try to topple it, it comes right
back to its original balance position, just like those little clown toys.
A very contrary, disobedient egg.
Properly presented, it can eggsasperate students who try to figure out what’s
going on. Practice till you are eggspert, and you’ll get eggcellent response
If you don’t want to bother with real eggs, consider using the colored plastic
eggs seen in stores around Easter-time, or the old L’Eggs (TM) containers
for panty-hose, or the small eggs silly-putty (TM) is sold in. Weights can
be glued at appropriate places inside.
Don’t omit a mathematical discussion of how the center of mass must relate
to the radius of curvature at the table-top to ensure that the egg won’t
roll to a different position. This is a good demo for illustrating conditions
ELECTROSTATIC INFLUENCE I. Charge a glass rod by rubbing it with silk and
bring it near the thin stream of water from a faucet. The stream will
be deflected toward the charged object. Charge a rubber or plastic
rod by rubbing it with wool or fur, and the water will also be drawn
toward the charged object.
Always do both cases, with external charged objects of different sign of
This demonstration shows that something charge-related is going on, and the
water itself must contain charges.
It does not, however, show that the molecules are charged,
or that they have a net charge. If they were, you’d expect attraction in
one case, repulsion in the other.
The molecules are polar; they have a permanent dipole moment. In the external
field they align themselves in that field, + ends in one direction along
the field, – ends in the other direction.
They why is there an attraction? Because the field is nonuniform. The field
exerts a stronger force on the molecule end near the external charged body
than it does on the molecule end slightly farther away.
This should be followed by discussion, with diagrams, showing the forces
and demonstrating that in either case the net force is toward the stronger
part of the field.
As always, be sure to sweat the details to get the maximum instruction from
these simple demonstrations. [DES]
ELECTROSTATIC INFLUENCE II. A charged comb attracts small scraps of paper,
or a piece of string or hair. Can it also affect a paper match delicately
balanced on a coin inside a glass? How does the electric influence pass through
the insulating glass? Touch the charged comb to the glass to find out. [DES]
EFFECT OF GAS DENSITY ON SOUND. Fill several balloons with different gases
such as air, carbon dioxide, natural gas, helium, and propane to about the
same pressure.Fix a whistle to be blown to a short piece of glass tubing.Note
the pitch as gas from the different balloons blows the whistle. [HG]
ELEMENTARY BATTERY. Show the emf produced between solutions of different
concentrations by using two copper discs attached to insulated wire and suspended
in a dilute copper sulfate solution, then drop a few crystals of copper sulfate
in to make the bottom layer more concentrated. Connect the electrodes to
a sensitive milliammeter or galvanometer. [HG]
ENERGY CONSERVATION. Suspend a bowling ball with a strong cord from the ceiling,
Draw the ball back against your nose, with your head against the wall. Release
it and stand nonchalantly awaiting its return. It cannot rise to greater
height from which it started. You are safe if you do not move or push
the ball during its release. [HG] Experienced (and daring) demonstrators
do this with the back of their head touching the classroom wall.
FALLING LEAKY BUCKET. A large Styrofoam (TM) cup is filled with water.
Punch a small hole in the side near the bottom and a stream of water shoots
out sidewise. Plug the hole with your finger before much has come out. Why
does the water shoot out this way? Because the weight of the water above
the hole causes the pressure just inside the hole to be above atmospheric
pressure, and the water near the hole moves from high pressure to low pressure.
What would happen if the cup were dropped from a considerable height? In
the cup’s frame of reference it and everything else are accelerating at the
same rate and are (in this frame) weightless, so the pressure everywhere
in the cup drops to atmospheric pressure. Then there’s no pressure difference
between the inside and outside of the hole, and no water shoots out. It’s
best to this outdoors, from and upper floor window, and particularly entertaining
when a Dean or department chairman is walking beneath. If done indoors, from
a balcony or high ladder, have it fall into a plastic wastebasket. [DES]
Richard DeLombard suggests a very nice extension of this demo. First, he prefers a water bottle with a hole punched in it, being easier to handle than a styrofoam cup. Do not cap the bottle. Now ask students what would happen if the bottle were carefully tossed upward without spin or tumbling? After it leaves your hand, will the water come out the hole? If not, what will happen, and why? This is best done outdoors. [Answer]
FINGER FRINGES. Observe a line-filament lamp through the space between two
fingers, the fingers being parallel to the filament. Vary the width of the
space until you see a pattern of fine lines between the fingers. Such
line-filament lamps are about six inches long, have a single, essentially
straight filament and are unfrosted. They are sold for use in store display
cases. This demo can also be made to work by looking at a long fluorescent
lamp from across the room. A backlighted hand may look as if you are seeing
an X-ray of the bones of the hand. Observe objects through a bird’s feather.
Here’s the puzzle. What are these fringes? Diffraction patterns? Very likely they aren’t. If
they were diffraction patterns they would show spectral colors. Too many “gee whiz” science demo books for children tell you that they are diffraction patterns. They lie. [Answer]
FISSION BUBBLE. Activation of a nucleus to cause it to fission may be simulated
by catching a soap bubble between two wire rings with handles. When caught,
puncture the top and bottom areas leaving a cylinder between the rings. Carefully
pull the rings apart, noticing the shape of the film, until it breaks in
two films over each circle. [HG]
FLAME DISCHARGE. Ionization in a flame can be shown by holding a lighted
match near a charged electroscope. Charged pith balls or balloons lose their
charge rapidly when a flame is brought near. [HG]
[DES] The figure shows a pulley arrangement called the “Fool’s Tackle”, for
only a fool would expect it to work. However, if the diagram is shown to
students (without the forces included) you can ask them what force would
be required to lift the load from the table: W, W/2, W/3, W/4, where W is
the weight of the load.
The answer is “none of these”. The upper movable pulley drops till it hits
the bottom one. The forces don’t add up. This illustrates the fact that you
can draw something on a blackboard that can’t work in nature. It also reinforces
the advice we give to students in lab: Do the analysis using known physical
principles and make a prediction before undertaking the experiment.
I’ve seen students struggle for a half hour to make this system work before
they ask for help.
In case you were wondering, the whole system can be inverted, with the free
end pulling up. It doesn’t work any better.
Some of us sneaky types like to treat this as a deception. We have the apparatus
set up with the load resting on the table. Then we pull on the free end and
the load rises. Best to do this after you’ve convinced the students that
it can’t possibly work. Of course, you have pinned or otherwise gimmicked
the upper pulley so it can’t turn. If the string slips on that pulley, coat
it with rubber cement.
GAMES. [HG] Games can make both learning and instruction a pleasure. Build
a puzzle of jumbled letters for other students to solve.An example:
CTVREO — has magnitude and direction
DSEPE — magnitude portion of velocity
OHNTPO — elementary light wave
GROWING SILVER CRYSTALS. Place a copper penny on the glass slide of a
micro-projector. Put silver nitrate solution around the penny and watch silver
crystals form on screen. Note the many peculiar characteristics they exhibit.
GYROSCOPE, LARGE. Weld bicycle axle nuts into one end of each of two iron
pipes. Screw the pipes onto the wheel axle for handles. This makes an excellent
gyroscope. It’s even better when the rim is weighted by winding it with iron
wire. Or fill the tire with sand for greater weight. [Be
very careful not to get your fingers in the whirling spokes, for the weighted
wheel has considerable inertia]. [HG]
GYROSCOPE BAT CIRCLES. While standing or sitting on a rotating platform,
turn around by swinging a baseball bat in circles over your head. Reversal
of the swing reverses the motion of your body. [HG]
[In this, and the following demos using a rotating platform,
consider the policy of your school before letting students do them. It’s
not unreasonable to have wrestling-mat pads surrounding the platform, and
have the platform surface padded as well. People become dizzy and have been
known to fall off the seat or platform.]
GYROSCOPE HOME RUN SWING. Show action and reaction by standing on the rotating
platform and swinging a baseball bat vigorously at a pitched ball. This should
be amusing. Do it outside, of course. [HG]
GYROSCOPE MOMENTS. Again on the rotating platform, pirouette. Hold heavy
weights at arm’s length, have someone rotate you slowly. When you are “up
to speed” bring the weights close to body. Explain the marked increase in
speed. [HG] [The speed increase can be great enough to disorient
an inexperienced person, causing them to lose balance and fall off the
GYROSCOPE PLATFORM. Construct a rotating platform from an automobile front-wheel
and spindle. Rigidness, coupled with small friction and small play in the
bearings is amazing. This is useful to demonstrate rotational inertia and
maneuvering in space. [HG] There’s great value in having each student do
this to feel the direction of the reaction torques. If this isn’t
practical, the demonstrator should describe in detail what direction the
wheel seems to “want to move” as one tries to make the wheel move in a particular
GYROSCOPE PRECESSION. Stand on a rotating platform holding a spinning gyroscope
wheel with its axis horizontal. Observe what happens when the axis is rotated
to a perpendicular position to the right? To the left? [HG]
HAIR RADIO TRANSMITTER. Combing dry hair near the aerial of an AM radio produces
static. [HG] You could call the static “long-hair” music, but that joke is
lost on younger people.
HEAT TREATMENT. The effect of heat treatment and tempering of metals can
be demonstrated by heating bobby pins to redness in a Bunsen flame. Dip one
heated pin in cold water to chill it. Allow the other pin to cool slowly.
Compare these pins with one that has not been heated, by bending each one.
HEAVY FINGER. [DES] This is an old demonstration, but
I think it is still worth doing and deserves special attention to its
presentation. Use an old-fashioned double pan balance. Don’t be tempted to
use an electronic balance, for the students must be able to see the
changes. A beaker of water is on one pan, balanced by weights on the other
pan. Propose this question to the audience: “Suppose I were to stick my finger
down into this water, being very careful not to touch the sides or bottom
of the beaker, what will happen?” If there’s any hesitation, or confusion
say, “It’s really an easy question! What will happen is that my finger will
“But that’s obviously not what I meant. Let me phrase the question more
carefully. Will the pan with the beaker move down, up, or would the scales
remain balanced when I insert a finger into the water?”
During the discussion, demonstrate that if you press a finger on that pan,
it goes down. If you press on the edge of the beaker, it goes down. “But
if I’m careful to touch only the water, not the beaker, what happens?” Refuse
to actually do it till serious discussion ensues.
Finally, do it. The pan and beaker move down. “Why?” you ask?” Don’t peek
at my answers until you’ve come up with at least two instructive answers
of your own. [Answer]
HIGH ROAD AND
LOW ROAD. [BERG] Two balls are propelled with equal velocity on two tracks.
The tracks start out on the same level, but one has a smooth drop to lower
level, then a rise to higher level. The total length is over a meter. Some
care must be taken to ensure equal firing speeds.
An alternate version uses two incline ramps at the left to achieve equal
velocities of the balls. I made one 5.5 feet long of steel construction set
parts (Meccano, Erector, Steel-Tech, Temsi, etc.). It includes two parallel
45° 10 inch long inclines at the start of the track to give the balls
equal initial speed. The lower level portion of the track is two feet long,
and is 5 inches lower than the upper track. The smooth, curved rise is about
a foot long horizontally.
Construction set model. The balls are posing at rest for the photo.
Which ball wins the race? The balls gain kinetic energy on the first incline,
the one with the longer incline gains an additional kinetic energy of mgh
compared to the other ball. h is the height difference between the horizontal
portions of track. So the lower ball is ahead of the upper ball on the horizontal
tracks, and having higher speed, continues to gain its distance lead over
the upper ball. On the final rise, the lower ball loses as much kinetic energy
(mgh) as it had gained early on, so on exit they have the same kinetic
energy and the same speed. But the distance advantage the lower ball gained
going at higher speed on the straight track ensures that it wins the race.
This is a version of the famous “Brachistochrone Problem” first solved by
Johann Bernoulli in 1696. It is discussed in most intermediate mechanics
books. The problem was to determine what curve of incline will get an object
from point A to point B in the least time, when A and B are at different
heights. The curve of least time is a cycloid. Such problems led to the
development of the calculus of variations. [Answer]
“HOLEY” WATER. Do molecules of water have spaces between them? Pour water
into a long test tube or graduate until it is three-fourths full. Then completely
fill it to capacity with alcohol. Place your palm over the top of the container
and invert it. Be careful that no liquid is lost as the water and alcohol
mix. Observe that the container is no longer full. Evidently some alcohol
has disappeared in water molecule holes. [HG]
HOT DOG WHISTLE. Tune two metal dog whistles to unison or absence of beats.
Heat one whistle with a flame. Beats reappear as the pitch of the heated
whistle rises. [Please don’t burn your lips!] [HG]
HOT ROD BALANCE. Drill a brass rod for a screw in one end. Insert a screw
about half way. Balance the rod at its center on a pivot. Throw off the balance
by moving the small screw out. Heat the other end of the rod and it comes
into balance again. [HG]
HYDROSTATIC SCALE. Weigh yourself by hydrostatic pressure. Use a hot water
bottle with a stopper fitted with about two meters of rubber and glass tubing.
Fill the bottle with water and connect the tubing so that it extends vertically.
Lay the bottle on the floor and cover most of it with a small board of known
area. Stand on the board and measure the increased height of the water in
the tube. Your weight is equal to the area of the board times the water pressure
increase. Calculate the water pressure by multiplying the density of water
(1 gram per cubic centimeter) by the difference in the water level height
when you stand on the scale. [HG]
HYDROSTATIC PUZZLE. [MG] A cork or wood ball floats on water. Add a layer
of oil to the water surface. Does the floating ball rise, or sink, when the
oil is added? Answer: It rises. Why?
IN CUP). Attach a ball to the bottom of a cup with an elastic band. When
the cup is dropped, what will happen to the ball? Will it stay outside the
cup? Will it jump inside the cup?
Answer: It jumps inside the cup. In free fall, in the cup’s frame
of reference, the ball is weightless, and it was its weight (the force of
gravity upon it) that was countering the upward tension of the rubber band
when the cup was at rest (as shown in the diagram). As the cup falls, the
tension force, unopposed, pulls the ball up and over the rim into the can.
INTERRUPTED PENDULUM. Show conservation of energy in a swinging pendulum
by noting that the bob returns to the same level each time. Place a peg or
obstruction below the point of suspension so that the arc of swing will be
changed to a shorter radius. Locate the peg at a point one half the distance
between the lowest and highest levels of the bob and then do it again with
the peg still lower. Explain why the bob loops over the peg. [HG]
INVISIBLE THWACK. [MG] Bend a playing card and stand
it upright. Stretch a rubber band on another playing card perpendicular to
it. The two cards are about a centimeter apart. The far end of the band is
pulled back and released. The standing card is knocked over, even though
you can’t see that anything touched it. The front end of the rubber band
actually moves forward, breaking contact with the card. But when does this
happen, and why?
Presentation: A more durable apparatus consists of two large nails
in a board, the rubber band being stretched over them, and a card of any
sort set upright a short distance (about 1 cm) beyond one of the nails. First
snap the band without the card in place. Ask students to describe the motion
of the band in detail. They usually will not suggest that the front of the
band never breaks contact with the nail. Raise the issue. Then do the card
demo to confirm your hypothesis. Some may still doubt that the band hit the
card, thinking perhaps that you jiggled the table or the board, or even blew
on the card to knock it over. If they don’t suggest this, suggest it yourself.
“How can we test the hypothesis that the band knocked the card over, when
our eyes can’t see it do that?” Substitute a sense that isn’t as easily
fooled as the eye. Have a student put a finger where the card was, to feel
the band hit the finger.
One answer is commonly seen and seems superficially plausible: When the band
is stretched, the front nail (B) exerts a force on it, but your hand balances
that force. When you release the band, there’s an unbalanced force of the
front nail (B) forward on the front end of the band. This gives a forward
impulse. The momentum of the band is forward, and its center of mass moves
forward faster than the band can relax to its unstretched position.
I must admit that I was taken in by this bogus answer. I had forgotten that
it takes some time for the front end of the band to “know” that the other
end has been released. That is, it takes some time for any physical effect
of the release to reach the front end.
But the real answer is more interesting. Loren Winters and Travis Williams
at the North Carolina School of Science and Mathematics have taken high speed
photos of the band in action. These may be seen at this
projects in high speed photography page, along
with other fascinating pictures of physical phenomena.
They did more investigation to answer these questions:
Does the forward motion from nail (B) begin only after the band is fully
relaxed or when the moving end hits the nail (A), or the compressional
wave from end (A) has reached (B)? What if nail (A) weren’t there?
They found that the pulse that knocks the card over occurs after the end of the band that was
pulled back reaches nail (A), and a compression pulse still moves forward past nail (B) toward the card.
In fact, only one supporting nail (B) is necessary.
This illustrates the principle that you should never give up thinking about
a problem just because you’ve gotten what you think is a plausible answer.
Even if you have a correct answer, there are always other ways to
arrive at it, and some of these may give you more insight.
The explanation of the SLINKY DROP demo is related
to this. Both are examples of a general principle. Changes in one part of
a system cannot affect another part of the system until some physical influence
passes from one part to the other. Such influences do not travel instantaneously.
One common source of mistakes in doing mechanics problems is to assume (without
thinking about it) that when the force is removed from one end of the rubber
band or spring that the other end “knows” about it instantly.
IMPULSE AND INERTIA MAGIC. Done with graceful flourish, this brings down
the house! The mechanics of friction, forces and inertia involved stimulate
interesting discussion. Set a glass two-thirds full of water about three
inches from the edge of a table. On the glass place a pie tin. On the pie
tin and directly over the glass place a spool on end. Place an egg (fresh
if you are confident) on the spool. With one foot on the bristles of a springy
broom, pull back the handle and aim at the pie tin. The spool rolls on the
table, the pie tin scoots to the floor, the glass and the water remain unmoved
on the table with the egg unharmed in the water.Note: The pie tin overhangs
the edge of the table. The table edge stops the broom’s forward motion before
it hits anything else. [HG]
The wood block must be at least as high as the edge of the pie tin.
Variations. Only your imagination limits the possible variations you
might try. With practice, you can snatch the pie tin quickly enough with
your hand, using a smooth arm motion. Don’t hesitate.
heavy balls are suspended one above the other by strings. When you give a
steady pull on the lower string, C, which string will break?
[Caution: your hand must get out of the way quickly so it
won’t be hit by a heavy falling ball. Some demonstrators fasten the lower
string to the center of a wooden dowel, then use both hands on the dowel
ends to jerk it down.]
Which string will break when you jerk the lower string? [BERG]
This is a variant of the demonstration using only one ball and two strings,
say the top ball and strings A and B in the diagram. When B is pulled slowly,
A breaks, because the tension in A is larger than that in B by an amount
equal to the weight of the ball. When B is jerked its tension rises almost
immediately to the breaking point, while the tension in A rises more slowly,
since it takes a short time for the heavy ball to move and stretch string
A enough to reach its breaking point. The reason for the time delay is that
in F=ma applied to that mass, m is large, so
a is small.
To really understand this demonstration one must remember that a string doesn’t
break until it is stretched (elongated) to its breaking point.
In these “inertia” demos involving jerks (including the tablecloth yanking
demo) one must consider the rapidity of the motion, and the fact that the
duration of the force can be smaller than the time it takes to move something
enough to cause a particular outcome. [DES]
Also see: Ehrlich’s comments [Ehrlich, Toast] p. 16, and Arons’ comments
on so-called “inertia demonstrations”. [Arons] sec. 3.22. Arons concludes
that “Without visualization of the stretching of the strings, students acquire
no understanding of the demonstration; they simply memorize, and repeat,
that it had something to do with ‘inertia.'” [<]
JERKS AND YANKS. [DES, MG] Quite a number of
demonstrations depend upon impulses of brief duration. The classic demo of
this type is the one in which a tablecloth is yanked from under a full table
setting, plate, silverware, glasses of water, etc. without toppling or spilling
anything and without moving the table setting to a significant velocity.
Use a silk or smooth rayon cloth, and be sure that the trailing edge has
no hem. It helps to roll up the edge you are pulling all the way to the edge
of the table before yanking it, helping to ensure that the cloth doesn’t
“bunch up” anywhere during the yank.
Of course each object does move a bit. They move during the brief time while
the tablecloth slides from under them. Then, this motion continues as they
are brought to a stop by friction against the tabletop. It is important that
both the cloth and the tabletop be smooth and low-friction. If the table
surface slows them too quickly, tall objects like glasses and candlesticks
can topple forward.
Silverware can cause problems in this demo, and is best avoided.
Often this demo is passed off with the explanation. “The objects are not
disturbed because of their inertia.” This explains nothing. When the tablecloth
is pulled slowly, everything moves along with it. If it is pulled more rapidly,
the wine glass topples. If it is pulled (yanked) very quickly, nothing is
seriously disturbed. Why?
The force an object experiences is that due to friction between it and the cloth,
and that friction is proportional to the normal force (equal to the object’s
weight) and the coefficient of friction. The coefficients of static and sliding
friction aren’t different enough in size to account for the outcome of this
The impulse given by the cloth to an object on it is Ft, where
F is the tension in the cloth and t is the duration of application
of the force. The impulse changes the momentum of the objects on the cloth.
F depends on the friction force, and this is nearly independent
of the speed of movement of the cloth. Since the friction is proportional
to the normal force, which is constant in all cases, the impulse depends
only on the time, and if the time is short enough the impulse is small. Therefore
it is the impulse that is the key to the understanding of these experiments.
One can also argue that the cloth is removed in a time shorter than that
required for the objects to accelerate much. This is really the same as the
explanation of the previous paragraph. But one must still emphasize the
importance of the fact that the normal force is independent of the horizontal
force and also independent of the time of application of the horizontal force.
As always, in elementary mechanics courses, draw the free body diagrams as
See: Ehrlich’s comments [Ehrlich, Toast] p. 16.
demos can also be done using a strip of paper and one object, if you haven’t
yet acquired a table setting and a silk or rayon cloth. An 8.5 x 11 inch
piece of paper will do. Place it under a beaker or glass of water. Pull slowly.
The glass moves along with the paper. Pull it nearly to the edge of the table.
With the glass in this precipitous position, say “I will now pull the paper
from under the beaker without spilling any water.” (Say “pull” not “yank”.)
Suspense! Then yank the paper. Follow up by using an empty plastic glass
and show that if the paper isn’t pulled quickly enough, the glass will
topple. Practice first.
Other objects may be used: a lighted candle, a smooth-edged coin balanced
on its edge, a smooth wooden ball. Try the coin trick with the coin in a
plane perpendicular to the yank, and also parallel to the yank. For a brisker
yank, hold the paper out horizontally from the table and do a downward karate
chop on it with your finger (Fig. d).
A little more suspense. Balance that glass of water, on the paper, sitting on your
head. Be sure the bottom of the glass is dry.
Then yank the paper. Practice, using a plastic tumbler. You have to
hold your body still while doing the yank. Easier: do this on your palm,
with a dollar bill under the glass on the edge of a table. Yank out the bill. (Dinner-table
I have three brass cylinders of different weights and sizes, the largest
being an inch in diameter. I set them on the paper. “Big inertia, medium
inertia and small inertia” I say. They are so “touchy” that the slightest
motion of the paper makes them roll. Then I do the karate chop move to yank
the paper from under them, and none of them even jiggle. So what does this
teach us about inertia? Nothing.
So, do any of these “demonstrate inertia”? All objects on the cloth or paper,
whatever their mass, seem to behave the same. Even a lightweight folded napkin
in the table setting behaves the same. So these aren’t clear
demonstrations of the property of inertia. But they are nice
demonstrations of impulse.
As one student observed, when asked to explain this, “If you apply a force
quickly enough to something, it doesn’t notice.” At least that’s a better
description of what happened than “It was because of inertia.”
LENZ’S LAW. Lenz’s Law may be demonstrated with any toy wheel of nonmagnetic
material and low friction attached to a convenient holder. The wheel should
have spokes for clearest understanding. Spin the wheel in air then between
the poles of a reasonably strong horseshoe magnet. Spokes cut lines of force,
the induced current field opposes motion. [HG]
THE CENTER OF GRAVITY. Start with your hands outstretched and palms facing
each other about a meter apart. Rest a horizontal stick, cane or metal pipe
on the index fingers of each hand. With your eyes blindfolded, slowly move
your hands together until the palms meet. Regardless of the starting position
of your hands, the center of gravity of the stick will be at the point where
your hands come together. [HG]
A meter stick is good for this demo. One can even tape a weight at some point
on the stick and it still works. A two-dimensional version: Use a large dinner
plate, with objects on it, perhaps even a glass of water. Use three fingers
spread wide to support it. Bring the fingers together! Your fingertips should
be dry. [DES]
A mathematical analysis of this can be found in Arnold Sommerfeld’s
Lectures on Theoretical Physics, Volume 1, Mechanics Academic Press,
1964, pp. 83-85. It’s a good exercise in forces, torques, and friction.
MAGNETIC WAVES. Suspend a bar magnet on a string. Rotate another magnet under
it to show transfer of magnetic energy. What changes the direction of the
poles? How can the change be effected without human movement. [HG]
MASS SPECTROGRAPH. Properties of alpha, beta, and gamma rays may be simulated
by propping a smooth board of about eight inches by twelve inches on an incline
and arranging a bin with a trap gate at the top so that three different sized
balls can be released to roll down the board. Place a strong magnet below
the board and just to one side of the gate. Note how each falling ball goes
into a separate bin because of the amount of deflection. The gamma may be
represented by a brass or aluminum ball, he beta would be the smaller of
the steel balls. [HG]
MATCH DISCHARGE. Rubber bands or strips can be tied together in bundles and
charged by stroking with fur or by other means. A lighted match near the repelling
strips will cause them to collapse. [HG]
MATCH HEAD DIVER. A Cartesian diver can be made with a Coca-Cola bottle full
of water and a match head. Continue cutting or breaking off the match stick
until the head barely floats. Thumb pressure on the mouth of the bottle makes
these little divers zip up and down in the bottle. [HG] Experiment with glass
bottles with secure plastic caps. Large medicine bottles with flat sides
can be pressed on the sides to make this work, since glass is slightly flexible.
There’s no surprise in doing this with plastic bottles. Presentation
note: Since so many plastic bottles are made to look like glass ones,
tap the bottle on the table to let the audience hear that it is glass.
MIRROR IMAGE REVERSED? Why does a plane mirror seem to reverse your image
left-right but not up-down? This frequently-asked question has value for
encouraging students to think more precisely and use words carefully. Why
should this question even be interesting? Where’s the “problem”? The discussion
shouldn’t be carried out entirely in the abstract. Have a large mirror to
demonstrate. Rotate the mirror around its normal as a student looks at the
image. The image doesn’t rotate. The mirror seems to be operating with axial
symmetry. Then why should the mirror treat up/down differently than right/left?
(Some students will even suggest gravity has something to do with it, so
have such students look in the mirror and lean their heads sidewise.) Have
one student look into the mirror while another stands behind the mirror looking
at the first student. Have each touch the top of their heads. Have each touch
their right ear. Perhaps have each wear something distinctive on the right
shows that a pair of plane mirrors at right angles behaves differently than
does a single plane mirror. Have two mirrors hinged (for ease of storage)
with a precise arrangement to hold them at perfect right angle alignment
for the demo. Curved cylindrical mirrors can be made by forcing plastic mirrors
into a slight curve. This may not seem like physics, but it is certainly
a good exercise in three-dimensional thinking.
NUT DROP. Tie six or seven metal nuts on a string at distances in proportion
to (1/2) gt2 where the time is 1, 2, 3, 4, etc. seconds.
Hold the string vertical and still and then let it drop. Note there is no
difference in the time intervals as nuts strike the floor. [HG] To get full
value from this demonstration, have another string with nuts tied on it with
equal spacings. Drop it first, to demonstrate that the sound of the impacts
with the floor do not occur at equal time intervals. Here’s a conclusive
test of the Aristotelian vs. Galilean laws of motion; a test accessible directly
to the senses without the need for timing devices. In a large room it’s worth
putting two pulleys near the ceiling to hold the strings of nuts until they
are released. Or use light threads over supports near the ceiling to suspend
the two strings of nuts, then cut the supporting threads. [DES]
OSCILLATING BEAM. [K&K] A heavy uniform bar or beam rests on top of two identical
rollers that are continuously turned in opposite directions, as shown. There’s
friction between the rollers and the bar, and it’s constant, independent
of the relative speed of the surfaces. Find the motion of the bar.
Follow-up question: What happens if the rotation direction of both wheels
This homework problem from Kleppner and Kolenkow may be demonstrated. I made
a model from Erector set parts. The rollers are 1 inch pulleys with thick
rubber o-ring tires. The rollers are about 6 inches apart, driven by a pulley
arrangement with a long rubber o-ring belt, driven by a standard Erector
set motor geared down to slow speed. The beam is a 10 or 12 inch angle girder.
Place the girder on the rollers so that its angle is like an upside down
V: ^. When the rollers turn, the girder oscillates back and forth with simple
harmonic motion, without falling off the rollers. This is just as predicted
by the mathematical analysis. Sometimes the friction is a bit erratic, but
the girder stubbornly refuses to fall off. I’ve had a model in a display
case with a button to activate the motor, and many people have tried to topple
the girder without success. Of course, if the motor were reversed, both rollers
turn the other way, and the girder smoothly moves in one direction and falls
Steel construction set model of the oscillating beam.
Some students argue that sometimes, once in a blue moon, the stopping position
of the girder would be just at a balance position, and when the motor is
started again the girder would just sit there without moving in either direction
as both wheels slid underneath it. It has never happened yet. This must be
telling us something profound about how nature works.
This is related to the “locating the center of gravity” demo in which you
support a meter stick with two fingers, then move the fingers together.
PAPER KETTLE. Boil water in a paper cup. The paper will not burn until the
water has boiled away. [HG]
The usual demonstration: Cover a thin wooden slat or shingle with
a sheet of newspaper except for a few inches of the slat that project beyond the edge
of the table. Hit the protruding part of the slat with a sharp downward blow
of a sturdy broomstick. The slat breaks without tearing the paper. [HG]
The usual explanation: The paper has been smoothed down over table
and slat. Some books simply say that
the pressure of the air downward on the slat is what secures it in place
on the table. If that were all there were to it, the paper would not be needed.
If the paper is carelessly placed and not smoothed down, this shouldn’t work,
according to the usual explanation. Any correct explanation must include the
fact that smoothing the paper helps prevent air from getting under the paper easily.
The paper is somewhat flexible, so when the shingle is struck, the portion under the paper
rises a bit, creating a space with very low air pressure. The force on the slat is now
unbalanced, with much greater force of air pressure above. If the slat breaks before
air has a chance to flow into the space under the paper, the demonstration is a success.
Consider the old experiment in which two of the rubber flat sink-stoppers
(about six inches diameter) are pressed together, then one attempts to pull
them apart. (The poor man’s Magdeberg hemisphere experiment. If you are even
poorer, use one stopper on a smooth tabletop.) If the stopper surfaces are not very smooth,
they pull apart easily. But if you sand them with very fine emery paper,
or moisten them, they are difficult to separate. Can we say it is simply
the air pressure that prevents them from separating? No. The fact that they are
flexible, allowing creation of a low pressure space between them, is essential.
The fact that air can’t easily get between them is also essential. So what
did Magdeberg demonstrate in 1654? He used iron hemispheres, and deliberately
used the newly invented air pump to remove most of the air between them.
Could we separate the two hemispheres
(or sink-stoppers) if they were surrounded by a vacuum?
But this demo of breaking of the slat is misleading. Demonstrate it using a
wooden yardstick lying on the table with a bit of overhang. Strike the overhang
with your hand, and the stick goes flying up in the air. Now replace the
stick and strike the overhanging portion forcefully with a broomstick. The
end breaks off. No newspaper is needed! I often do this with old strips of
lath of the kind used in old houses under plaster walls. This may also be
playing a role in the usual demonstration.
Explanation of the paperless breaking demo: Air pressure plays no role
in this version. This works best if the portion of the lath lying on the
table is long compared to the overhang. The blow to the overhanging end gives
an impulse of short duration. This stresses the overhanging end, as it starts
to bend. The internal stresses cause a wave of stressed wood to move toward
the table. When this reaches the edge of the table the wood is forced down
against the table edge, and an upward reaction force from the table acts
on the wood. This force and the downward force of the broomstick act as a
couple, tending to rotate the portion of the yardstick extending beyond the
table. The more massive portion of the yardstick has not yet received any
of the stress wave, and has not yet been acted on by any forces tending to
cause it to move up away from the table. Crudely, “It doesn’t yet know anything
has happened.” The yardstick breaks before the longer portion has a chance
PENCIL POINT BALANCE. To demonstrate center of gravity outside of a body,
and the criterion for stability, borrow two pocket knives from students.
Push the blades firmly (but carefully) into a pencil near the sharpened end
with the handles beyond the point of the pencil. Balance the pencil point
on your finger. Since the system’s center of gravity falls below the point
of balance, the system is stable. [HG] Today, students are not allowed to
carry knives in schools. How times have changed! When I was in school in the
1940s and 50s, nearly every boy carried a pocket knife, for whittling, playing
mumbledy peg, trimming wooden slingshots made from tree branches, and sharpening pencils.
And I never heard of a case where a knife was used as a weapon. [DES]
PINHOLE EFFECTS. An interesting conversation piece can be made from an empty
35 mm film can. In the center of one end punch one hole with a sharp needle.
About the center of the other end punch three pinholes at the corners of
an equilateral triangle about two millimeters apart.Look through the one
hole and see the three holes. Look through the other end at the one hole
and explain what is seen. Label the box “Drunk-O-Meter” and list the following
directions: 1 hole—sober, 2 holes—nipping, 3 holes—dog drunk,
4 holes or no holes at all—dead drunk. [HG]
Submerge a beaker full of water in a large water filled container or fish
tank. Invert the beaker so its open end is down. Invert a second beaker and
submerge it so that air is trapped inside. Pour air from one beaker into
another, pouring up. Note the fluid behavior of the gas. [HG]
POURING CARBON DIOXIDE. Construct a series of three 5-cm steps that will
fit into a wide mouth jar. Set a lighted candle on each step. Slowly pour
carbon dioxide gas from an open container into the jar. Carbon dioxide is
heavier than air. As it settles it extinguishes the candles one by one starting
with the candle at the lowest level. There are many ways to generate carbon
dioxide. Try mixing some vinegar with bicarbonate of soda. [HG]
PRECISION IN ADVERTISING. Encourage students to think, speak, and write more
precisely. Illustrate by using a meaningless advertising slogan: “The
Rolls-mobile is bigger and better than (a) a kiddie car (b) a freight car
(c) last year’s model. [HG]
The little toys with four metal vanes inside an evacuated glass globe are
readily available. They are sometimes called “light-mills”, but the proper
name is “Crookes Radiometer.” Many books give incorrect explanations of how
these work, and the correct explanation may be more than you wish to deal
with. Before you demonstrate one, read Bill
Beaty’s How does
a light-mill work?.
RATE OF HEAT CONDUCTION. Three students, each holding a rod of a different
substance in a flame, will demonstrate the difference in conductivity of
heat by their object from the flame. Use about the same sized rods of iron,
aluminum, glass, copper. [HG]
REACTION OF THE ROAD. Place a plank on rollers (doweling). With a string,
tie a small cart to one end of the plank and stretch a long rubber band between
the cart and the other end of the plank. Add weights to the cart to increase
its mass. Burn the string to release the system. The road goes one way, the
cart goes another. [HG]
RETINAL AFTER EFFECTS. Draw a circle in the center of a piece of white paper
with colored crayon. Stare at the circle at arms length for a time, then
look at a blank wall. A circle of the complimentary color appears on the
Select a large spool and wrap several turns of ribbon or cord around it.
Place the spool on a table so it can roll when the free end of the ribbon
is pulled out from the spool bottom. Observe the direction that the spool
rolls when the ribbon is pulled straight up and when it is pulled at other
angles closer to the horizontal. With a little practice, the spool can be
made to roll in either direction as the ribbon angle is changed. Encourage
students to explain the phenomena using terms such as torque, friction, and
vector direction of force. [HG]
At a particular
angle of pull the spool slides without rolling, if the angle is maintained
as it moves. Ask students to determine, from physics principles (forces and
torques), precisely how that angle can be predicted from the nature of the
spool. Answer: The angle is such that a line extended along the ribbon passes
exactly through the point of contact of the spool with the table, therefore
the ribbon exerts no torque about that point. The force due to friction and
the gravitational force mg also pass through that point, so the net
torque about that point is zero, and no rotation can occur around that point.
The case where the spool rolls toward you as you pull on the ribbon often
seems counter-intuitive to students. The spool is moving faster than the ribbon pulling it. But on reflection, isn’t it equally counter-intuitive that in other cases the spool
rolls opposite the direction you are pulling? This demonstrates once again that
our naive intuition often misleads us. More advanced students might want to work out the energy conservation implications of these cases.
A variation, to challenge the better students: Find a suitable sized cylindrical
solid object. Tape one end of a length of cash register tape to it and wrap
the tape around it’s circumference. Now pull on the tape. The cylinder rolls away from you,
as expected. Now bring the tape to the table top, and if you are very, very,
careful, pulling slowly, the cylinder slides toward you without rolling. Doesn’t
this seem to defy what we learned above about the angle of pull? What’s going
on here? The tension of the tape exerts a horizontal force on the cylinder,
but that’s matched by the friction force, so there’s no net horizontal force,
and no acceleration. The only torque on the cylinder is the tension of the
tape, with a lever arm equal to the radius of the cylinder. That torque should
produce a constant angular acceleration. But we observe no angular acceleration,
and no angular velocity. Is some physics being violated here? (A large roll
of cash register tape is fine for this demo if you can still find one.) [DES]
Challenge trick: When you’ve gotten the feel of this, tell students
that you can actually make the solid cylinder roll toward your hand. Use a
long lecture table, and slowly accelerate the system, so the cylinder slides
without rolling. Then suddenly stop. The cylinder’s inertia causes it to
roll toward your hand. [DES]
SEEING THE SUN BEFORE SUNRISE. The fact that one may see the sun while it
is still below the horizon can be simulated by looking at a penny at the
bottom of a bowl filled with water.Note that the penny cannot be seen over
the rim of the bowl unless there is water in the bowl. When the sun first
appears in the morning, it is still our of sight below the horizon. Refraction
of the sunlight by the atmosphere makes the sun appear higher than it really
INVERSION. Print with capital letters the word TITANIUM DIOXIDE. Use a red
pencil for the first word and a blue pencil for the second. View both words
through the side of a test tube filled with water. Only the red word looks
SELECTIVE LIGHT SCATTERING. Demonstrate the effect of the sun setting through
the dust-laden atmosphere. Add five grams of sodium thiosulfate and 5 mL
of concentrated hydrochloric acid to a liter of water in a clear container.
Shine a light through the solution and on to a wall or screen. Observe the
changes as the colloidal sulfur forms. Scattered blue light can be seen in
the solution at a ninety degree angle from the beam.On the screen or wall
the spot slowly changes from white to yellow, to red, and then is finally
is blacked out completely. [HG]
SHADOW REFRACTION. Place an object on the bottom of a metal pan so that its
shadow may be measured. Fill the pan with water and remeasure the shadow.
Refraction is evident if the pan, object, and light source are kept stationary.
SINGING FLAME VARIATION. Hold a four foot 1-1/2 inch glass tube vertical.
Insert in the bottom end at a predetermined resonance point a heavy disc
of wire gauze. Heat the wire gauze with burner, then remove the flame and
hear a phenomenon. [HG]
SINGING TUBES. A straight metal blow pipe connected to a gas supply is fixed
in an upright position on the demonstration desk and lighted.A thirty to
sixty centimeter glass tube of large diameter is lowered over the flame until
at a certain position a sound is heard. [HG]
SKY HOOK. Cut a four inch piece of wire from a coat hanger.Bend one half
inch back on one end so that a leather belt will slip in the hook. Rest the
free end of the wire on a finger tip. The belt and wire will hang out in
space without apparent support underneath. [HG]
SLINKY DROP. [BERG] Stand on the lecture desk for extra
height. Hold a SLINKY (TM) spring at one end. The other end shouldn’t reach
the floor. Ask what will happen if you release the upper end. Of course all
will agree that the center of mass of the unsupported spring falls with acceleration
g, and that the spring begins to contract when you let go of it. But what
are the relative motions? Among the possible outcomes:
- The lower end immediately falls with acceleration less than g.
- The lower end rises to meet the rest of the spring, till the spring contracts
fully, then the whole spring falls with acceleration g.
- The lower end rises to meet the rest of the spring as it is contracting.
- The lower end remains at rest, waiting for the rest of the spring to contract
on its way down.
The outcome surprises students. The lower end remains at rest at constant
height until the rest of the Slinky closes completely, then the whole thing
falls to the floor. The center of mass of the spring falls with acceleration
g. In the center of mass frame of reference; the spring collapses
toward its center of mass.
Note that the acceleration of the upper end of the spring is initially
approximately 2g. When the spring is hanging, the center of mass is
not at its midpoint, but is lower because each part of the spring must support
the weight of everything below, so the separation of the coils is greater
in the upper part than in the lower part. Does this affect the outcome? Why,
or why not?
But without getting mired in these details, we know that the lower end cannot
react until information reaches it from the top end. At the instant the upper
end is released, the lower end “doesn’t yet ‘know’ anything has changed.”
No physical influence from the upper end reaches it immediately.
think this has something to do with gravity, or with the non-linear stretch
of the Slinky. Not so. The same Slinky spring can be strung over a taut
horizontal wire or strong nylon cord (about 3 meters long for a plastic Slinky.
A weight (W) attached to one end hangs over a pulley. The other end is pulled
to stretch the Slinky, holding the weight in equilibrium. That end is released.
The weight does not begin to fall immediately, but only when the Slinky is
nearly collapsed. (The end may be attached to a string, which is then cut.)
For more instructional value, show the students the speed of a compression
pulse along the Slinky. With the Slinky stretched, compress a small portion
near one end and then let go. The pulse travels to the other end slowly enough
to watch. The same thing happens when one end is released. The weight begins
to fall only when the compressed portion travels from one end of the Slinky
to the other end. So long as the coils at the left end of the Slinky have
the same spacing they had initially, the tension there is the same as it
was initially, thereby holding the weight (W) in static equilibrium.
A follow-up question comes to mind. If you attached a weight to the
lower end of the suspended spring how would this affect the oucomes?
Surprisingly, it doesn’t. The suspended weight still doesn’t begin to
fall until the compression pulse reaches it.
Related experiment: INVISIBLE THWACK.
SOAP BUBBLES AND SOAP FILMS. [Dick and Rae] This recipe is from Richard B.
Minnix and D. Rae Carpenter. Use Joy diswashing liquid (no other brands work
as well). 140cc Joy, 300cc glycerine, and 450cc water. Approximately 1:2:3
ratios. Let the solution age a week before use for best results!
SODA-STRAW WHISTLE. [MG, DES] If you can still find paper soda straws,
try this. Flatten one end. Snip off the edges of that end to form two free
flaps of paper. Blow in that end. The paper acts like the double reed of an oboe.
Flatten the flaps more (or less) as necessary to produce the sound. Different
lengths give different pitch. Start with a long one, and snip off lengths
with scissors, making it successively shorter. When it gets shorter than
an inch or so, be careful not to snip your nose with the scissors.
Find another slightly larger diameter straw to slip over the first to make
a slide-whistle. Cut two to such a length that when played together you hear
beats. Mark one in advance with the proper lengths so you can cut successively
at the marks and produce a perfect ascending scale. Cut several, properly
tuned, to play a simple tune. The opening of “Jingle Bells” requires only
We farm boys used to make goose-feather whistles.
Snip off and discard the portion with the feathers, leaving only about a
two inch piece. Make a diagonal cut at the closed end, raising up a portion
of the shank so it looks like a miniature clarinet mouthpiece. Blow into
it for a high-pitched sound. It can be concealed in the mouth. Also, its
length can be extended with a soda straw or other tube.
A BOWL OF WATER. Submerge a mirror in a bowl of water as shown. When sunlight
falls on the water surface at a low angle, a spectrum will be cast on the
ceiling. Place this near a sunny window of the classroom. [MG]
SPECTRUM FROM A BALL POINT PEN. Some brands of ball-point pen have a clear
plastic barrel with hexagonal cross section. Pilot © pens do. When these
catch the sunlight they cast a nice spectrum, since the angle between alternate
facets is 60°. You don’t need to remove the ink tube from the pen.
Save these pens for lab use. [DES]
Such a pen may also be used in the classroom with an overhead projector.
Mask off all but a narrow 1mm slit on the projector light table. Notice that
only a narrow strip (1 or 2 cm wide) of the projection lens has light passing
through it. Place the pen in this strip of light (and parallel to it)near
the projector lens, and adjust it so that the spectrum it produces is at
minimum deviation. Narrow the slit if necessary to get purer colors. [DES]
Of course a long prism, or diffraction grating, or holographic diffraction
grating can be used with the overhead projector to produce a spectrum, but
the ball-point pen is far less expensive and the result is very good. [DES]
Question 1: Why does this spectrum form a curved rainbow-like arc
on the wall? The answer to this has no connection whatever to the
fact that a rainbow is curved in the sky.
More questions: As you rotate the pen, the spectrum moves through
different angles of deviation from the unaltered beam from the projector.
But there’s a distinct position where the deviation is least, called the
minimum deviation angle. Why is this? When the spectrum is at minimum
deviation, the light rays pass through the prism symmetrically, making equal
incident and emergent angles. Why? Why is the dispersion (spread of colors)
greater when the deviation is larger than minimum? What has this to do with
SPECTRUM FROM A PROJECTOR. For classroom demonstration an overhead projector
may be used to cast a large circular spectrum on the ceiling.
Obtain a disposable plastic glass, such as those used for cocktails at parties.
These glasses have sloped sides. Nearly fill it with water. Cut a hole in
an opaque paper just a bit larger than the bottom of the glass, and place
this mask on the easel of the overhead projector, with the glass sitting
in the center of the hole.This arrangement casts a circular rainbow on the
ceiling. The size of the hole in the mask may need adjustment. In some cases
the mask is not needed. Block the light on one side of the plastic glass
to demonstrate that the path of the light is upward at an angle into the
sloping side of the plastic glass, through the water, then out the surface
of the water on the other side of the glass. This is especially effective
in a fully darkened room. If the motor of the projector vibrates the water
surface, the spectrum jiggles beautifully. [DES]