Forces in Motion

15" balloons filled with either H (Hydrogen) or He (Helium) 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 handle 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

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.

Alcohol Rocket

A 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 handle 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 its 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 counterclockwise. 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 counterclockwise 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 sharp edges so that the amount of the professor's 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 professor's 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 sledgehammer to smash through all three bricks.