H and He Balloons
15" balloons filled with either H or He are popped with an 8" "torch"
lighter. This is used as an exercise in the scientific method. In science,
we make educated guesses based on observations, experiments are performed
to test those guesses, the guesses are modified if necessary, and the process
continues. Students are shown the balloons, and after noting that whatever
is in the balloons must be lighter than air, the students are asked what
might be in them. If mentioned, hot air can be discounted since there is
not an obvious heat source. Heat must be continuously be added to a hot
air balloon to keep it hot. Once choices are narrowed down to H and He,
the students are asked to come up with an experiment to tell the difference.
This results in the "flame test", since H is very flammable, while He is
not.
Coke Float
Something will float if it is lighter than whatever is around it, or,
in other words, if it is less dense than whatever is around it. The same
amount of stuff in a larger volume is less dense, and the same amount of
stuff in a smaller volume is more dense. The H and He balloons floated
because they were less dense than air, wood floats in water because it
is less dense than water, we float in water because we are less dense than
water, and battleships floats in water because they are less dense than
water. Even though battleships are made of steel, there is a lot of empty
space inside so that on the average it is less dense than water. Classic
Coke will sink in water, while Diet Coke will float. You can joke that
the diet coke floats because it has fewer calories. Actually, Coke sinks
because sugar water is more dense than water, while Neutrasweet is not.
Toilet Paper Tug
Two rolls of toilet paper, one full and the other almost gone, are
placed on a rod so that they are free to rotate. Inertia is discussed.
Inertia is a measure of how difficult it is to get something to move. We
use weight and mass to measure inertia. The heavier something is, the more
inertia it has, and the harder it is to get it to move. A very sharp or
fast blow applied to something with a lot of inertia is more likely to
break it rather than move it. The same blow applied to something with less
inertia is more likely to move it. It is very easy to rip off one or two
squares of toilet paper with a quick jerk of one hand from a full roll
without having it spin and unravel because of it's large inertia. However,
it is almost impossible to do this with a roll that is almost gone. Because
of it's low inertia, it is much easier to move.
Tablecloth Tug
A silky tablecloth is draped over one end of a smooth tabletop with
several plates, bowls, wine glasses, candlesticks, silverware, etc., on
the tablecloth. With a quick tug the tablecloth is pulled out from under
the table settings without having them move much. This all relies on the
inertia of the table settings, and the low friction between the table,
the tablecloth, and the table settings. Heavy table settings with bottoms
as smooth as possible are used. If you wish to try this be sure to remove
the hem from the tablecloth on the edge that must pass under the table
settings. A straight back, and slightly downward tug works best.
Weights on Strings
Large weights are each suspended by a single strand of breakable string.
A strand of this string is also attached to the bottom of each weight.
The weights are not attached to each other in any way. Students are
asked which string will break first: the one above the weights, or the
one below, when the strings below the weights are pulled. Which one
actually breaks depends on how the string is pulled. If the string
is pulled very fast, the string below the weight breaks first every time,
while if the string is pulled slowly, it breaks above the weight every
time. This is because of the inertia of the weights. A quick
pull does not give the weight a chance to move. In order for the
string above the weight to get tighter, the weight must move down a bit.
If the pull is fast enough, it gets much tighter below the weight than
above, and that is where it will break. If the string is pulled slowly,
it gets tighter above the weight, for the weight does have a chance to
move, and the string above has to support the weight and resist the tug.
Dowel Snap
Light, thin nails or pins are driven into each end of a 4', 1/4" hardwood
dowel and the heads of the nails or pins are removed. The dowel is placed
between two wine glasses so that the pins rest on the rims of the glasses.
The wine glasses are placed on small boxes on top of chairs. The dowel
is broken in the center with a sharp blow from a rod, without moving the
wine glasses much. This works because of the inertia of the dowel, and
the low friction between the pins and the glasses.
Egg Drop
Three eggs are balanced on top of three 5" tall tubes of rolled paper
on top of a pizza tray that is placed over three tall glasses filled with
water. Each of the eggs and tubes are positioned directly over one of the
tall glasses. A sharp blow to the pizza tray with a broom will knock the
tray out from under the tubes. The tubes will be knocked out of the way
by the lip of the pizza tray, while the eggs, with their larger inertia,
will drop straight down into the glasses of water.
Pencil Through Wood
A 4'x2'x2' plexiglass cage is used to shield the audience from this
demonstration. A sharpened pencil is shot down a steel tube towards a 1/8"
thick piece of plywood with compressed CO2 from a modified fire extinguisher.
The pencil will usually penetrate the plywood and be stopped about halfway
through. If a flat-ended unsharpened pencil is used, it will blast all
the way through the plywood every time. The sharpened pencil does not work
as well because it pushes the wood to the side as it passes through. It's
impact is spread out over a longer time than that with the blunt pencil,
and the wood that is pushed aside presses back on the pencil with a considerable
force. This is how nails hold wood together so well. The pressure on the
sharp pencil from the creates a huge amount of friction which will rapidly
slow the pencil down. The blunt pencil strikes the wood with a very sharp
blow, so fast that the wood breaks instead of moving aside. A neat circular
hole is knocked in the wood and the blunt pencil sails right through.
Egg Toss
Student volunteers are challenged to throw an egg as hard as they can
into a sheet that is supported by two long wood dowels. Unless they hit
one of the dowels, the egg cannot be broken. Just as inertia is a measure
of how hard it is to get something to move, momentum is a measure of how
hard it is to stop something that is moving, or to change it's motion.
It is much harder to stop a baseball moving at 60 mph than one moving at
10 mph. However, it is almost impossible for a human to stop a Mack truck
moving at only 3 mph. the momentum of something depends on it's inertia
as well as it's speed. The momentum of an object can be changed by applying
a force to that object over a period of time. A large force applied quickly
can have the same effect as a small force applied over a longer time. If
you play catch with eggs, you want to bring the egg to a stop slowly by
cupping the egg with your hands and allowing them to swing, so that the
force you apply to the egg at any one time is not large enough to break
it. If you throw an egg into a wall, it is brought to a very quick (and
gushy!) stop, and the force on it is very large. In the demonstration,
the sheet has a lot of "give" so that the egg is brought to a stop in a
comparatively long time, so that the force on it at any instant is not
enough to break it.
Hinged Board
A 6' 1"x4" board with a hinge near the middle is clamped to a ladder
so that the other end dangles straight down. Students are asked how much
weight must be hung from a hook near the end of the dangling board in order
for the board to "stay up", or horizontal. After some comic attempts with
larger and larger weights, a sledge hammer is hung through a loop that
is placed over the hook so that the sledge handle touches the board near
the end and the head of the sledge is past the hinge. With this the board
will stay horizontal. One explanation is that the center of mass of the
sledge and board are actually on the clamped side of the hinge. A more
correct explanation relies on action equaling reaction. If a sledge hammer
is held up by a point about 6" from the end of the handle, it takes a large
downward force to the end of the handel to hold it horizontal. The board
applies that force, but there is also an equal upward reaction force of
the board, large enough to hold it and the sledge hammer horizontal.
Push Me - Push You
Two students are asked to stand facing each other on separate wheeled
carts. The students push off from each other and they both move back.
It does not matter which student did the pushing, or even if both of them
pushed, because for every action there is an equal reaction in the opposite
direction. Each student experiences the same exact force as the other
student. So when a Volkswagen collides with a Mack truck in a fender-bender,
each vehicle experiences the exact same force, but in the opposite direction,
even though the truck is so much more massive. This works with a
big student pushing off of a small student. Each experiences the
same force, but the smaller student will roll farther because of her/his
smaller mass. If the Volkswagen and the Mack truck had identical
bumpers and supports, each would have the same amount of damage, even though
the motion of the truck is hardly changed, while the Volkswagen will be
knocked backwards.
Bowling Ball Pendulum
A pendulum, up to 20 meters (65 feet) in length, made of steel cable
and a 9 kilogram (20 pound) bowling ball, is used to discuss conservation
of energy. The pendulum is mounted to a sturdy support as high as
possible near the center of the room. A professor places his back
to a wall while standing, or sitting on top of a ladder and brings the
bowling ball up to touch his nose with the cable tight. This gives
the ball gravitational potential energy. This energy came from the
work that the professor performed to bring the ball up from its resting
place beneath the support to the height required to touch his or her nose.
Once the professor releases the ball, it loses its potential energy and
gains kinetic energy as gravity performs work on the ball to speed it up.
At the bottom of its arc, all of the ball's energy will be kinetic.
This energy is then converted again to potential energy as gravity performs
work to slow the ball. If the amount of energy the ball loses to
air friction and motion of the support is small, the ball will swing back
very close to the exact height from which it was released, again touching
the professor's nose.
Torque Meter
If your want to rotate something, where you push is just as important
as how hard you push. When you open a door, you want to push as far away
from the hinges as possible. The torque meter consists of a 2' long 2"
dowel joined at 90 to a 4' long 3/4" dowel with hooks 6" apart on one side
so that they are perpendicular to both dowels. Students are instructed
to firmly grasp the 2" dowel with both hands while a weight is moved from
one hook to another. It gets more and more difficult as the weight is moved
farther out. The force applied to the torque meter is the same, but because
the force is applied at different places the "torque", or the ability to
rotate an object, is different. A lever is a machine to increase the torque
by allowing more "leverage", or a larger distance from the pivot point
to the point where a force is applied.
LED Axe Toss
A giant lopsided axe made form stiff foam is thrown across the room.
Because of its lopsidedness, it flips around as it flies. A bright LED
located at the center of mass of the axe is turned on before the axe is
thrown again, but with the lights out. With the LED on it is quite clear
that the center of mass of the axe moves in a smooth arc without flipping
around.
Centripetal Ball
When something goes around a curve, people often talk of the centrifugal
force pushing outward. If you are in a car turning to the right, the centrifugal
force is to the left. This is not really a force at all. Although it is
useful to think of it as a force on occasion, the "centrifugal force" is
just the tendency of things to move in straight lines. As the car turns
right you want to go straight, but the car shifts to the right around you,
and so it feels as if there is a force on you to the left. For something
to move in a curve, there must be a force on it to keep it from moving
in a straight line. This force is called the centripetal force, and it
is always directed towards the center of the curve. To illustrate this,
a big styrofoam ball is swung in a horizontal circle by a thread. The centripetal
force is supplied by the tension in the thread and is directed towards
the center of the circle. A razor is brought up to cut the thread at specific
point. The ball then flies in the direction it was going right before the
thread was cut, "tangent" to the circle. If there as a centrifugal force
on the ball at all times, we would have expected the ball to fly straight
out and away from the person swinging it, in a "radial" direction, 90 from
the direction the ball actually went.
Centripetal Wine glass
It is not too difficult to swing a bucket full of water in a vertical
circle without spilling any water. In this demonstration, a full wine glass
is placed on a square platform of smooth clear plastic. A cords are attached
to each corner of the platform and are joined together in a knot 3' above
the platform. The platform and wine glass are swung in a vertical circle
without spilling or flinging the wine glass. It is the tendency for the
wine glass and its contents to go in a straight line that allows it to
seemingly defy gravity. The centripetal force provided by the tension in
the cords is large enough to create enough friction to hold the wine glass
in place.
Water Rocket
For every action there is an equal and opposite reaction. Whenever
you push on something, it pushes back with an equal force. All rockets
work on this principle. In a rocket, burning fuel, or exhaust, is forced
out the back of the rocket at high speed. The backward force on the exhaust
by the rocket is equaled by a forward force on the rocket by the exhaust,
and so the rocket goes forward. A water rocket made from a two liter pop
bottle is mounted on a guide wire and pumped up with a bicycle pump. When
a catch is released, the high pressure inside forces water out the back
of the rocket at high speed. This in turn propels the rocket forward.
Alcohol Rocket
Another flashy action-reaction demonstration uses an alcohol rocket.
A few ounces of isopropyl alcohol (95% or more works best) are poured into
a 5 gallon water cooler jug. With one hand capping the top, the jug is
shaken to fill it with the alcohol fumes. The jug is placed on a slight
incline, the hand removed, and a flame is brought to the opening as quickly
as possible. The gas inside the jug will burn, and expand rapidly from
the heat and the combustion products. The pressure will force flaming gas
backward out the opening at a high speed, which will in turn force the
jug forward. This is spectacular with the lights out. This demonstration
requires extreme caution. A fire extinguisher is necessary. The rocket
will shoot flaming liquid out along with the gas. Students must be cautioned
not to try this or any similar experiment on their own. A smaller but faster
rocket can be made with a 2 liter pop bottle. Drill a 1/8" touch hole near
the bottom of the rocket. Cover the hole with a finger as you shake 1/2
ounce of alcohol in the bottle. Plug the mouth with a cork and light the
touch hole. The expanding, burning gas will blow out the cork and fling
the rocket at a very high speed.
Compressed Gas Go Cart
A professor sits on a wheeled cart with a cylinder of compressed gas,
probably CO2. Gas is shot out of the cylinder one way, and the professor
and cart are propelled in the opposite direction as a further illustration
of action and reaction.
Clay on a Stick
A 3 lb glob of clay is stuck on a 4' dowel, about 6" from one end.
Students are asked which would be easier: to balance the dowel vertically
with the clay near the top, or with the clay near the bottom. A volunteer
is asked to try it both ways. It is far easier with the clay near the top.
It is also much easier to wave the dowel back and forth with the ball of
clay near your hand than with it near the end. Just as inertia is a measure
of how difficult it is to get something to move, "rotational inertia" is
a measure of how difficult it is to get something to rotate. The same amount
of stuff near a pivot point has a much lower rotational inertia than the
same stuff farther away. It is much easier to balance something with a
larger rotational inertia because gravity has a harder time tipping it
over, giving the balancer more time to react when it starts to tip.
Rotating Stool
Ice skaters are well known to go into incredibly rapid spins. They
do this by changing their rotational inertia. They start spinning with
their arms and one leg sticking away from their body. They then lower their
rotational inertia by bringing their arms and leg back closer to their
body. As you spin on ice with an arm sticking out, your arm moves faster
than your elbow because it moves in a larger circle in the same amount
of time. It also has more energy. If you bring your arm back in close to
your body, you have to speed up to keep that energy. Student volunteers
are asked to sit on a low-friction rotating stool, and to hold weights
out at arms length. The students are spun slowly and told to move their
arms in and out. The smaller their rotational inertia, the faster their
spin, and visa versa. Angular momentum depends on the rate of spin
and the rotational inertia. In this case where very little energy is lost
to friction, angular momentum is conserved.
Bicycle Wheel
Students are given a bicycle wheel with a weighted rim and handles
on the axle. The wheel is much easier to move about when it is not spinning.
The faster it spins, the more angular momentum it has, and so the harder
it is to get it to change it's motion. This is why it is much easier to
balance on a moving bike than on a stationary one. Also, students may be
surprised that the wheel does not want to move in the directions they expect.
This is also due to it's angular momentum, but can be understood if the
wheel is thought of as separate chunks connected to the axle by a light
rigid rod. If you hold the wheel out vertically in front of you so that
chunks on the top are moving away from you and chunks on the bottom are
moving towards you, the net force on the top chunks is downward toward
the axle, while the net force on the bottom chunks is upward. These are
centripetal forces provided by tension in the spokes, and necessary for
circular motion. If you try to twist the wheel sharply clockwise, chunks
on the top are given a force to the right, while chunks on the bottom are
given a force to the left. The net force on the top chunks is then down
and to the right, while The net force on the bottom chunks is up and to
the left. The forward moving chunks on the top will then move down around
the circle and slightly to the right, while the backward moving chunks
on the bottom will move up around the circle and slightly to the left.
So it is not surprising that the wheel jerks to the right if you try to
jerk it clockwise, and similarly, it will jerk to the left if you try to
jerk it counter clockwise. This is why it is easier to "corner" on a bike
if you lean into the curve. This effect can be demonstrated dramatically
by spinning the wheel vertically, holding one handle up by a long cord,
and then letting go of the other handle. Instead of tipping over, the wheel
will start to spin horizontally while staying vertical.
Another good demonstration is to step onto or sit on a rotating stool with a spinning wheel. If you tip the wheel one way, you will rotate the other way. You can also hold a non-spinning wheel horizontal while on the rotating stool. Start spinning the wheel one way, and you will spin the other way. This illustrates conservation of angular momentum, but it can also be understood by thinking of action and reaction. If you apply a clockwise push to the wheel, it will apply an equal counter clockwise push to you.
Hoop vs. Wheel Race
A thin metal hoop and a solid cylindrical wooden wheel are the same
size and weight. They are both rolled down an incline from the same height
in a race. The wheel will always win because it has a lower rotational
inertia. More of it's weight is closer to it's axis than with the hoop.
It takes more energy to roll the hoop, so with the same energy, the wheel
will roll faster.
Soup Can Race
A can of Cambell's Chunky Beef Stew is rolled down an incline in a
race vs. a can of Cambell's Chicken Broth. Both cans are the same size,
shape, and about the same weight, but the Cambell's Chicken Broth will
win every time. This is because the broth can is liquid inside, while the
stew can is more solid. When the broth can is spun, much of the liquid
inside does not spin as fast as the outside of the can, if at all. It takes
less energy to rotate the broth can, so it will roll faster than the stew
can.
Flying Buzz-Saw
A loop of light chain is mounted on a wheel attached to a hand drill.
The wheel is spun at a high speed and then the chain loop is pushed off
of the wheel. The chain loop will keep its circular shape because
of its rotational, or angular, momentum, even if there are obstacles placed
in its path.
Shattered Glass Walk
A 4 meter (12 foot) long trough is filled with shattered glass to a
depth of 5 cm (2 inches). Even though the shards of glass are extremely
sharp, a professor can walk barefoot through the trough because the force
per unit area is small. There are enough of the sharp edges so that
the amount of the professors weight on any one sharp edge is not enough
to break the skin. The glass is deep enough so that if the professor
walks slowly, any sharp points that stick up above the others are pushed
down by his or her foot before all of the professors weight is shifted
to that foot.
Bed of Nails
A "bed" of nails (ISU's own "Puncture-Pedic") was made by pushing nails
through holes drilled through two plywood boards. A demonstrator can sit
or lie on the nails without excessive discomfort because the amount of
his weight on any one nail is not enough to hurt. This demonstration of
force per unit area is coupled to one on inertia by stacking three concrete
patio blocks on the chest of the demonstrator and then smashing them with
a sledgehammer. The three blocks have enough inertia so that they will
break before they have a chance to move. Much of the energy in the swing
of the sledgehammer goes into breaking the blocks. Also, the impact on
the demonstrator is spread out over the area that the last block has in
contact with his chest. Also, the impact is spread out over the time it
takes for the sledge hammer to smash through all three bricks.