How Things Work
Instructor Resources
The most effective way to illustrate the physics concepts encountered in a How Things Work course is with in-class demonstrations. While many physics departments or schools already have collections of demonstrations that they use with various introductory physics courses, I have found myself using a number of unusual demonstrations in How Things Work. Here are some of the demonstrations that I have used over the years. Within this collection, you will find old standards as well as some surprises. In addition to these demonstrations, you should remember that, whenever possible, you should inspect and disassemble the objects that you're discussing so that the students can see for themselves what's inside them and how they work.
1. Pulling a Tablecloth Out From Under Dishes
Description: You pull a smooth silk tablecloth out from under a place-setting, leaving the dishes essentially unaffected.
Purpose: To demonstrate that inertia tends to keep stationary objects stationary.
Supplies:
1 silk tablecloth (no hem on at least one side)
1 smooth topped table, just large enough for the place-setting
1 relatively smooth-bottomed plate
1 knife
1 spoon
1 fork
1 short wineglass with a smooth, lip-less bottom
red wine (or disappearing ink: about 1/4 tsp. of phenolphthalein in 1 liter water, with just enough sodium hydroxide—about 1/16 tsp.—to turn it pink. When exposed to air, carbon dioxide gradually deactivates the sodium hydroxide and renders the mixture colorless.)
Procedure: Place the tablecloth on the table and arrange the dishes as you would at dinner. Align the wineglass with the fork and make sure that you have room to pull the tablecloth out and down without hitting anything. Also make sure that there is no hem to catch the dishes as the tablecloth slides out from under them. Grip the edge of the tablecloth firmly so that your right hand is aligned with the knife and spoon and your left hand is aligned with the fork and wineglass. This alignment will help minimize bunching of the fabric as it slides under the objects. Now jerk the tablecloth smoothly and swiftly out from under the dishes. A slight downward motion will help ensure that you don't lift the dishes upward and flip them. Don't pause—every instant is crucial. You want to minimize the time during which the tablecloth is pulling on the dishes. If you move quickly enough, the dishes will barely move.
Explanation: The dishes remain in place because of their inertia. Since they experience only modest frictional forces from the tablecloth, and because the time during which those forces are exerted on them is very brief, the dishes continue doing what they were doing: they remain stationary on the table. The tablecloth leaves them behind.
Follow-up: Students can try this procedure by placing some objects on a sheet of paper and then snapping the paper out from under the objects.
Description: A puck glides steadily in a straight line after being pushed.
Purpose: To show that an inertial object follows a straight line path at a steady speed.
Supplies:
1 frictionless balloon- or dry-ice-powered puck (Mattel used to make a battery-powered Airpro air hockey puck that was perfect for this demonstration, but they dropped it in 1991. I still use it.)
1 flat, level surface
Procedure: Make sure that the surface is as flat and level as possible. Use paper shims to level it until the puck can remain almost stationary when left alone. Then give the puck a gentle push and let it glide. It should travel at constant velocity. Show that, once free of horizontal forces, it always travels at constant velocity.
Explanation: The cushion of gas below the puck allows it to slide virtually without friction. As a result, it can follow its inertia in horizontal directions. It moves at a steady pace along a straight line path, as required by Newton's first law of motion, so long as you aren't pushing on it.
Description: A banana dropped from one hand is cut in half by a knife held in your other hand.
Purpose: To show that an object's inertia can keep its velocity from changing while you work on that object.
Supplies:
1 banana (relatively ripe and easy to cut)
1 sharp kitchen knife
Procedure: Hold the banana in one hand and the knife in the other. Drop the banana and, with a rapid sweeping motion, slice the banana in half with the knife. The two halves of the banana should continue falling almost together to the floor.
Explanation: The banana's inertia keeps it from accelerating horizontally. Although gravity makes the banana fall (and is thus a nuisance in this demonstration), there is nothing to make the banana begin moving horizontally. When you swing the knife through the banana, the banana's inertia keeps it in place. Although the knife does exert a small horizontal force on the banana, that force lasts for such a short time that it causes almost no change in the banana's velocity. The banana is simply sliced in half and continues falling to the floor.
Follow-up: How does this effect relate to mowing the lawn with a rotary mower? or to operating a kitchen blender?
Description: You throw a banana horizontally at a knife held in your other hand and the banana cuts itself in half.
Purpose: To show that an object's inertia will keep it moving at constant velocity in the absence of outside forces.
Supplies:
1 banana (relatively ripe and easy to cut)
1 sharp kitchen knife
Procedure: Hold the knife upright in one hand and throw the banana at the knife with your other hand. (Don't cut yourself! You can also mount the knife upright on the table if you like.) The banana should be flying freely and horizontally when it encounters the knife blade. If the blade is sharp, the banana relatively soft, and you've thrown the banana hard enough, the banana will smoothly slice itself in half and continue on its way.
Explanation: The banana's inertia keeps it moving steadily forward as it encounters the knife. Since the knife barely pushes on the banana, the banana travels through the knife and is sliced in half.
Description: A puck accelerates in the direction of a force on it.
Purpose: To show that force and acceleration are in the same direction.
Supplies:
1 frictionless balloon- or dry-ice-powered puck (Mattel used to make a battery-powered Airpro air hockey puck that was perfect for this demonstration, but they dropped it in 1991. I still use it.)
1 flat, level surface
Procedure: Make sure that the surface is as flat and level as possible. Use paper shims to level it until the puck can remain almost stationary when left alone. Give the puck gentle pushes in various directions and show that it accelerates in the direction you push it. You can show that acceleration backward (against its velocity) slows it down. You can also show that making the puck travel in a circle takes a steady inward (centripetal) force.
Explanation: The gas cushion under the puck keeps friction from influencing its motion. It can thus respond to forces in accordance with Newton's second law of motion.
Description: You perform a series of movements that show the students the differences between velocity and acceleration.
Purpose: To help reduce the confusion between velocity and acceleration.
Supplies:
None
Procedure: One of the best ways I've found to illustrate velocity and acceleration is to walk (and even run) about while pointing in the direction of my acceleration (if any). I start from rest, then accelerate toward the right, then maintain constant velocity, then accelerate toward the left, and come to rest. There are many variations on this idea and I use a variety of them in my lectures. I also show accelerations toward a center, so that I travel in a circle. It's particularly important to show the students that you can accelerate without changing speed (by changing directions instead).
Explanation: Acceleration isn't as easy to see as velocity. However, by watching a person's velocity and the changes in that velocity, the students can begin to perceive accelerations.
Follow-up: Walk around as before and have the students point with their hands in the direction of your acceleration or velocity.
Description: A bowling ball is much harder to accelerate than a baseball.
Purpose: To show how mass affects acceleration.
Supplies:
1 bowling ball (or another massive ball)
1 baseball (or another low-mass ball)
Procedure: Put each ball on a table and give each a brief horizontal push. Try to exert the same force for the same time in each case. The baseball will accelerate to much higher speed than the bowling ball. Point out that this behavior has nothing to do with weight, since it would happen even in the absence of gravity. It has only to do with the masses of the two balls.
Explanation: The bowling ball has a much greater mass than the baseball. Since an object's acceleration is inversely proportional to its mass, the more massive ball experiences the least acceleration.
Follow-up: Consider cases of objects (e.g., vehicles) that are hard to start or stop because they are quite massive.
8. A Ball Falling Downward
Description: A ball dropped from rest accelerates downward and eventually hits the floor.
Purpose: To show that gravity exerts a downward force on a ball, causing it to accelerate downward. This acceleration continues indefinitely.
Supplies:
1 baseball (or another small ball)
Procedure: Hold the ball still in your hand and let go. The ball will fall, moving downward faster and faster as it accelerates in response to its own weight. Point out the this acceleration continues all the way to the floor. To prove that the students already know that this continuing acceleration takes place, have them consider whether they would mind if you dropped the ball on their hands from 2 centimeters. Then ask them whether it would still be OK from 2 meters. If they think about those questions, they'll realize that they know that the ball continues to pick up speed as it falls and is thus accelerating the whole way down.
Explanation: The earth's gravity gives the ball a weight—that is a gravitational force in the downward direction. When only this downward force acts on the ball, the ball accelerates downward. Its velocity increases in a downward direction from zero when it starts to a rapid downward velocity when it hits the floor.
Description: Two different balls, having different masses, are dropped from equal heights at the same time and they hit the floor at the same time.
Purpose: To show that all objects fall at the same rate (in the absence of air resistance).
Supplies:
1 marble (or another very small ball)
1 baseball (or another small ball)
1 bowling ball (optional)
Procedure: Hold both balls (the marble and the baseball) in your hands and drop them simultaneously from the same height. They will hit the floor simultaneously. Discuss what would happen if you dropped a bowling ball as well (it's probably not a good idea to actually drop the bowling ball).
Explanation: Although the baseball has more mass than the marble, the baseball also has more weight. That means that while the baseball is harder to accelerate than the marble, gravity also pulls more strongly on the baseball. In fact, gravity's pull on each ball is exactly proportional to its mass, so that the baseball receives exactly the right pull to make it accelerate together with the marble. No matter what object you pick, its weight (the gravitational force it experiences) will be just right to make it accelerate together with the marble. In short, all objects at the earth's surface accelerate at the same rate, in the absence of air resistance.
Description: A ball tossed directly upward rises and falls, always accelerating downward in response to its weight.
Purpose: To show that a ball that is free in the air is always falling—always accelerating directly downward.
Supplies:
1 baseball (or another small ball)
Procedure: Toss the ball directly upward and catch it as it returns to your hand. Discuss the direction of acceleration (steadily downward the whole time). Discuss what force(s) the ball experiences (only the downward force of gravity…nothing else—a fact that the students will be extremely slow to accept completely). Discuss what drives the ball upward to its peak height (the ball's inertia alone).
Explanation: A ball falls from the moment it leaves your hand, even if it's initially heading upward. While its velocity may be upward, its acceleration is directly downward and caused only by the ball's weight. There is no "force" pushing the ball upward and there is no special change in the ball's acceleration that occurs when the ball reaches maximum height. The falling process is very smooth as the ball's velocity gradually shifts from upward to downward.
Description: A ball tossed upward takes the same time to rise to its peak as it does to descend to your hand.
Purpose: To show how symmetric a ball's flight is as it rises and falls.
Supplies:
1 baseball (or another small ball)
Procedure: Toss the ball straight up and catch it as it returns to your hand. Count aloud the time it takes to rise to its peak and the time it takes to descend to your hand. Make sure that you count the time intervals during the rising and falling periods and not the beginning and ending moments—if you aren't careful, you will over-count by one on the way up. With a little practice and a high ceiling, you can get to 3 (or even 4) intervals on the way up and the same number on the way down and it can be pretty clear that the time up is the same as the time down. Practice first.
Explanation: The ball's upward speed when it leaves your hand is the same as its downward speed when it returns to your hand. Since the acceleration due to gravity is constant, it takes the same amount of time for the ball to lose its upward speed as it rises as it does for the ball to gain its downward speed as it descends.
Description: Two balls fall to the floor simultaneously, even though one ball starts with a horizontal velocity and the other starts from rest.
Purpose: To show that a horizontal component of velocity has no effect on a ball's vertical motion.
Supplies:
2 baseballs (or other small balls)
Procedure: Hold both balls in your hands at the same height. Drop one at the same moment that you throw the other horizontally. If your timing is good and your throw is truly horizontal, the two balls will hit the ground at the same moment. There are commercial gadgets that use springs to drop two balls in this manner, but I've had good results just doing it by hand. You could also make your own gadget from a springy wooded stick that supports one ball while you bend it with your hand and then strikes another ball horizontally when you let go of it. The first ball should lose its support and begin falling while the second ball should be knocked horizontally off its support and also begin falling.
Explanation: Since the force of gravity acts only in the vertical direction, it has no effect on a ball's horizontal component of velocity. Moreover, the ball's vertical component of velocity increases steadily in the downward direction, regardless of its horizontal component of velocity. In short, the two balls fall at the same rate and hit the ground simultaneously because their horizontal components of velocity don't affect their vertical motions.
Description: A ball tossed at several angles travels the farthest downfield when it's thrown at roughly 45 degrees above horizontal (neglecting air resistance).
Purpose: To show that both the horizontal and the vertical components of velocity are important to downfield distance.
Supplies:
1 baseball (or another small ball)
Procedure: Throw the ball directly upward at a particular speed and show that, while it stays in the air a long time, it doesn't travel downfield. Then throw the ball horizontally at the same speed, just above the table, and show that, while it moves rapidly downfield, it doesn't stay in the air long enough to travel downfield very far. Finally, throw the ball at 45 degrees above the horizontal and show that, because it stays in the air for a moderately long time and moves downfield at a moderate rate, it travels downfield rather far.
Explanation: The ball's vertical component of velocity determines how long the ball stays in the air and its horizontal component of velocity determines how effectively it uses that time aloft to move downfield. Given a fixed starting speed for the ball, the throw that moves it downfield most effectively is at 45 degrees above horizontal (although this neglects the effects of air resistance).
Follow-up: Have the students throw water balloons on an open field and get a feel for what initial angle allows them to throw the balloons the farthest (thanks to C. Conover for this idea).
14. Dropping an Egg on the Floor
Description: An egg shatters when it's dropped on the floor.
Purpose: To show that the egg and the floor exert equal but oppositely directed forces on one another.
Supplies:
1 raw egg
Procedure: Hold the egg in your hand and describe what is going to happen when you drop the egg. Point out that at the moment the egg reaches the floor, its inertia will tend to carry it downward and into the floor. Because the floor and the egg can't occupy the same space, they will begin to push against one another very hard—the egg will push downward on the floor to try to move the floor out of its way and the floor will push upward on the egg to try to stop it from descending. The forces will be equal in magnitude but opposite in direction. Now drop the egg.
Explanation: The egg shatters because the force exerted on it by the floor is (1) very large—because the floor must bring the egg to rest very quickly and must give it a large upward acceleration—and (2) exerted on only a small portion of the egg's surface. Since the floor's force on the egg is exert only on one part of the egg, most of the egg doesn't accelerate and continues downward. Only the part of the egg that touches the floor accelerates upward and the rest of the egg soon overtakes it. The egg deforms and shatters.
Description: Two people pulling in opposite directions on a book leave that book motionless.
Purpose: To show that objects accelerate in response to net force, rather than in response to individual forces.
Supplies:
1 book (avoid a rope, because its non-rigidity leads to complications.)
2 people
Procedure: Have the two people pull on opposite ends of the book so that the book remains motionless. Note that, since the book isn't accelerating, the net force on it must be zero. The forces from the two people and the force of gravity (the book's weight) are canceling one another perfectly.
Explanation: Anytime an object isn't accelerating, the net force on it must be zero. If you can identify a force pulling that object in one direction, you can be sure that there are other forces pulling it on the opposite direction.
Description: Measuring an object's weight with a spring scale.
Purpose: To show that a surface exerts an upward force on an object exactly equal to the object's weight and that a scale reports the upward force it exerts on an object.
Supplies:
1 heavy object
1 spring scale or bathroom scale
Procedure: Put the object on the scale so that the two remain stationary. Point out that the net force on the object is zero—it's not accelerating. Identify the two forces on the object: its downward weight and the upward support force that the surface exerts on it. Since the upward support force on the object must cancel the object's downward weight, they must have equal magnitudes. The scale reports the upward force it's exerting on the object, so it reports the object's weight.
Explanation: It doesn't matter what pushes on the scale—the scale merely reacts to any force exerted on it from above by pushing back with an equal but oppositely directed force. In this case, the downward force on the scale is the object's weight and the scale pushes up with a force that's equal in magnitude to that weight.
Follow-up: What is the scale reporting when you push down on it with your hand?
Description: You raise, hold, and lower a weight to identify those times when you do work—when you transfer energy.
Purpose: To show that work is done only when a force is exerted on an object and when that object moves a distance in the direction of the force.
Supplies:
1 object (a heavy ball or weight)
Procedure: Hold the object motionless at chest height and then raise it gradually upward over your head. Discuss the direction of the force you are exerting on it and the direction in which it moves (both upward). Point out that you are doing work on the object and that you are transferring energy to the object.
Now hold the object motionless over your head and again discuss the force and direction of motion (upward and none, respectively). Point out that you are doing no work on the object and that its energy isn't changing.
Now gradually lower the object back to chest height and repeat the discussion. This time you are doing negative work on the object (or, equivalently, it's doing work on you) and it's transferring energy to you.
Finally, walk at constant velocity across the room and discuss the fact that you are not doing any work on the object because the force and distance are at right angles to one another. However, be aware that people will wonder about the starting and stopping moments, when you are doing work. Discuss that starting and stopping process.
Explanation: To do work on an object, you must exert a force on it and it must move in the direction of that force. When you do work on the object, you transfer energy to it. Since energy is a conserved quantity, your energy goes down whenever you transfer energy to another object.
Follow-up: People will wonder about why you get tired when you hold a weight motionless above your head, since you aren't doing any work on it. The answer is that your muscles are inefficient and turn food energy into thermal energy even when they do no work on outside objects…in effect, they just burn the food to produce thermal energy. This internal conversion tires you out. They will also wonder what becomes of the energy returned to you when you lower the object back to chest height. The answer is that it becomes thermal energy in your muscles—they just aren't able to turn this energy into a more useful form.
Description: Energy you transfer to a ball becomes gravitational potential energy as the ball moves upward. It then becomes kinetic energy as the ball falls.
Purpose: To show two forms of energy: gravitational potential energy and kinetic energy.
Supplies:
1 baseball (or another small ball)
Procedure: Slowly lift the ball upward from a table. Point out that something about it is changing—it's acquiring a stored form of energy: gravitational potential energy. Point out that you are providing this energy. Then drop the ball and let the gravitational potential energy transform into kinetic energy. The ball's height decreases so its gravitational potential energy decreases. However, its speed increases so its kinetic energy increases. Overall, its energy remains constant. By the time the ball reaches the table again, its gravitational potential energy is gone and all of the energy you transferred to the ball has become kinetic energy.
Explanation: Energy is a conserved quantity. If you don't exchange energy with the ball, then its energy won't change. Thus as it falls, its total energy can't change but the form that energy takes can and does.
Description: A spring scale is used to show that the force needed to keep a cart from rolling down a ramp is much less than the cart's weight.
Purpose: To show that the ramp helps to support the cart's weight so that a small force is needed to support the cart or pull it steadily up the ramp, and that the work done in raising the cart to a certain height doesn't depend on whether the ramp is used.
Supplies:
1 adjustable ramp (or a board and some books)
1 cart
1 hanging spring scale
Procedure: First use the spring scale to weigh the cart. Now place the cart on the ramp and use the spring scale to determine how much force is required to keep the cart from rolling down the ramp. Show that this force is less than the cart's weight and that it becomes even less as the ramp's slope decreases—the ramp is helping to support the cart's weight. Show that the force needed to keep the cart from moving at all is the same as that needed to keep the cart moving steadily up the ramp because in both cases the cart isn't accelerating. Note, however, that only in the latter case are you doing work on the cart. Now discuss how far you must travel along the ramp to lift the cart upward a certain height. Note that while it takes much less force to pull the cart along the ramp than to lift it straight up, you must travel farther along the ramp to reach a certain height than you would were you to lift the cart straight upward.
Explanation: Overall the product of force times distance traveled, in short the work you do on the cart, doesn't depend on how you raise the cart upward. With or without the ramp, you must do the same amount of work to raise the cart to a particular height.
20. Rotation about Center of Mass
Description: Balls and other objects tossed into the air spinning rotate about their centers of mass while their centers of mass fall.
Purpose: To show that an object's motion can often be separated into its center of mass motion (translational motion) and rotation about its center of mass (rotational motion).
Supplies:
1 basketball (or another large symmetric ball)
1 stick (or, ideally, a juggler's club)
Procedure: Spin each object on the table to show that it naturally rotates about a special point—its center of mass. Then throw each object into the air while spinning to show that it continues to rotate about its center of mass while that center of mass flies through the air like a normal falling object.
Explanation: Gravity effectively acts at the object's center of gravity (which coincides with its center of mass). As a result, the object experiences no torque in flight and continues to rotate freely. At the same time, the object's center of mass falls under the influence of gravity.
Description: A ball wobbles rapidly back and forth after being thrown upward while spinning.
Purpose: To show that an isolated object rotates about its center of mass.
Supplies:
1 beach ball (or another inflatable ball)
1 rubber balloon filled with sand (not stretched)
1 duct tape
Procedure: Tape the sand-filled balloon firmly to the surface of the inflated beach ball. Now give the ball a spin as you toss it in the air. The ball will wobble wildly back and forth about its center of mass.
Explanation: Putting sand on the ball's surface shifts its overall center of mass toward the sand. When isolated, the ball will no longer rotate about its center—it will rotate about this new center of mass.
Description: A long stick balances when it's supported at its center of mass.
Purpose: To show that being balanced means experiencing zero net torque, and not necessarily being horizontal or motionless.
Supplies:
1 long stick with a hole drilled through its center of mass (a meter-stick with a hole through its center is ideal)
1 supported shaft that fits reasonably well inside the stick's hole
Procedure: Support the stick by placing it on the shaft. Show that the stick will remain motionless and horizontal if you start it that way. Since it's not undergoing angular acceleration, it's clearly experiencing zero net torque. Now tilt the stick away from horizontal and show that it still balances—it still experiences zero net torque and doesn't undergo angular acceleration. Finally, spin the stick about the shaft and show that it rotates steadily (neglecting friction and air resistance, which gradually slow it down)—so it still balances.
Explanation: Being balanced means only that an object experiences zero net torque. It may or may not be horizontal or motionless. All that you can be sure of is that it will remain motionless if it starts that way and that it will continue to turn steadily if it starts that way.
Description: A bowling ball is much harder to spin than a basketball.
Purpose: To show that an object's angular acceleration depends both on the torque it experiences and on its moment of inertia.
Supplies:
1 bowling ball (or another massive ball)