Description: Illustrate the Coasting Behavior Made Possible by Skates or a Scooter

Purpose: To remind students that, without friction, objects coast

Procedure: Show first that when you start motionless, you tend to remain motionless. Then show that once you're moving, you tend to continue moving steadily in a straight line. Although you'll revisit these ideas many times, you do well to show it first in the context of the official object.

Explanation: Skates and scooters free the rider from friction, at least along one direction. The rider is then able to exhibit the coasting behavior predicted by Newton's First Law.

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.

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 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.

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? or a spinning-blade coffee grinder?

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.

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 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.

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 student riding a hovercraft 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.

Procedure: When released from rest, the student should remain at rest. After being pushed horizontally, the student should coast horizontally at constant velocity.

Explanation: The cushion of air allows the hovercraft to coast horizontally, obeying Newton's first law of motion.

Description: A puck accelerates in the direction of a force on it.

Purpose: To show that force and acceleration are in the same direction.

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: A student riding a hovercraft accelerates when another student pushes him/her

Purpose: To show the relationships between force, mass, and acceleration.

Procedure: Have one student push another student around on the floating hovercraft. Have riders of different masses undergo accelerations. The goal is to illustrate that the rider only accelerates when pushed and that the acceleration is proportional to the push and inversely proportional to the rider's mass. .

Explanation: Without friction affecting its motion, the unpushed hovercraft coasts. But the hovercraft accelerates when pushed and obeys Newton's Second Law.

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.

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.

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.

Description: A student finds a hidden mass by shaking boxes rather than lifting them

Purpose: To show that mass is associated with resistance to acceleration.

Procedure: Before class, hide the bowling ball or other massive object in one of the boxes and put both boxes on rollers. Tell the class that there is a massive object hidden in one of the boxes and they are to figure out how to find that object without looking inside the boxes and without lifting the boxes (and thereby weighting them). Someone will eventually shake the boxes, using the rollers to eliminate friction, and discover which one contains the object.

Explanation: By shaking the box, you measure its mass. The more massive box is the harder to shake.

Description: The blindfolded instructor figures out which cart holds the student

Purpose: To show that mass is associated with resistance to acceleration.

Procedure: Set the two carts side-by-side and put on the blindfold. Have a student quietly stand on one of the carts. Without lifting the carts, you carefully push each cart back and forth and then announce which cart holds the student.

Explanation: By shaking the carts, you are measure their mass. The cart bearing the student is more mass and is therefore harder to shake.

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.

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: 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—even when it is rising.

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 up and forward rises and falls, always accelerating downward in response to its weight even as it coasts down field.

Purpose: To show that a ball that is free in the air is always falling—always accelerating directly downward—even when it is traveling in an arc.

Procedure: Throw the ball in arc. Discuss the direction of acceleration (steadily downward the whole time). Discuss why the balls path is an arc: that it rises and falls as a falling ball while coasting steadily down field.

Explanation: A ball falls from the moment it leaves your hand, even if it's initially heading upward and to the side.

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).

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 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.

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.

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 down field 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 down field distance.

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 down field. Then throw the ball horizontally at the same speed, just above the table, and show that, while it moves rapidly down field, it doesn't stay in the air long enough to travel down field 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 down field at a moderate rate, it travels down field 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 down field. Given a fixed starting speed for the ball, the throw that moves it down field 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).

Description: Corks are shot out of a liquid-nitrogen or dry ice pop gun at various angles and the ones fired at about 45° above horizontal travel farthest.

Purpose: To show that both the horizontal and the vertical components of velocity are important to down field distance.

Procedure: Hold the tube upright with gloves and goggles on and add some liquid nitrogen or dry ice. Carefully insert a cork and tamp it in gently. Let it fire (safely) straight up. Then repeat with another cork and let that cork fire (safely) almost horizontally (don't let the liquid nitrogen pour out). Finally, repeat and fire the cork (safely) at about 45° above horizontal. That last cork should travel farthest.

Explanation: The cork's vertical component of velocity determines how long the cork stays in the air and its horizontal component of velocity determines how effectively it uses that time aloft to move down field. Given a fixed starting speed for the cork, the shot that moves it down field most effectively is at 45° above horizontal (although this neglects the effects of air resistance).

Description: A ball sits on the table, not acceleration and therefore experiencing zero net force.

Purpose: To illustrate an object experience both its weight downward and a support force upward, with those forces summing to zero net force.

Procedure: Simply set the ball on a table and consider why it isn't falling. It is evidentially experiencing exactly the right upward force to balance its downward weight so that the net force on it is zero.

Explanation: The ball "negotiates" with the table so that table exactly supports its weight, no more and no less. When the ball settles down to motionlessness, the upward support force from the table is exactly equal in amount but opposite in direction to the ball's weight.

Description: Two students push or pull on a cart with equal but oppositely directed forces and the cart doesn't accelerate

Purpose: To show that an object can experience several forces at once and that the object accelerates in response to the sum of those forces, the net force on the object.

Procedure: Have two student push or pull on the cart with the spring scales. The cart will accelerate according to the vector sum of those two forces. For example, if both student push (or pull) in opposite directions equally hard, the cart won't accelerate.

Explanation: The two forces sum to give the net force. If that sum is zero, the cart will not accelerate.

Description: Each student tries to push on his/her neighbors with them pushing back

Purpose: To show that forces always occur in equal but oppositely directed pairs.

Procedure:Have students push on one another in their seats. Have one try to push and the other avoid pushing. Of course, it can't be done.

Explanation:As observed by Newton's third law, any force one object exerts on a second gives rise the an equal but oppositely direct force that the second exerts on the first. Try as they like, the students can't avoid exerting equal but oppositely directed forces on one another. Some years my students do this little experiment eagerly and unselfconsciously, but other years they won't do it out of embarrassment. It all depends on the lead in and I haven't figure out how to guarantee success.

Description: Two students use spring scales to push on another another. Try as they might, they can't make the two scales read differently.

Purpose: To show that the forces two students exert on one another are equal but opposite

Procedure: Have two students push on one another using spring scales. Each student has his/her own scale and the goal is to have one student's scale read differently than the other. No matter how contorted they get, they just can't make the scales read differently, indicating that the force student A exerts on student B is exactly equal in magnitude to the force student B exerts on student A.

Explanation: As observed by Newton's third law, the force student A exerts on student B must be equal but opposite to the force student B exerts on student A. The scales demonstrate that equality.

Description: Two people pulling equally hard in opposite directions on a book don't cause the book to accelerate.

Purpose: To show that objects accelerate in response to net force, rather than in response to individual forces.

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:Two students use spring scales to push on another another, while one is moving toward or away from the other. Try as they might, they can't make the two scales read differently.

Purpose:To show that the forces two students exert on one another are equal but opposite, even when one is moving!

Procedure:Have two students push on one another using spring scales. Each student has his/her own scale and the goal is to have one student's scale read differently than the other. No matter how contorted they get or how fast the one on the cart is moving toward or away from the stationary one, they just can't make the scales read differently, indicating that the force student A exerts on student B is exactly equal in magnitude to the force student B exerts on student A.

Explanation:As observed by Newton's third law, the force student A exerts on student B must be equal but opposite to the force student B exerts on student A. The scales demonstrate that equality.

Follow-Up:Students often misunderstand Newton's third law and imagine that because it predicts equal but oppositely directed forces, that those forces must cancel and no acceleration can occur. Have the two students push on one another while one stands on the cart and observe that the student on the cart accelerates. Discuss the fact that only one force of the Newton's third law pair acts on the student on the cart, so he/she accelerates.

Description: An egg shatters when it's dropped on the floor.

Purpose: To show that the equal but oppositely directed forces that an egg and the floor exert on one another can become enormous while the egg is accelerating upward.

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. These two 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: 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.

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: A ball lifted high in the air has gravitational potential energy. The same ball dropped or swung around has kinetic energy.

Purpose: To show two forms of energy: gravitational potential energy and kinetic energy.

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. Discuss the fact that the ball now has something rather threatening about it that it didn't have before. 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. Note that this kinetic energy also makes the ball more threatening than it was while motionless at the same height. Both the elevated ball and the fast-moving ball have the capacity to do things; to do work.

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: You raise, hold, lower, and carry 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.

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: 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.

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.