Interactive Lecture Demonstration

Prediction Sheet—Momentum

Directions:  Click here to download the Prediction Sheet. Write your name at the top to record your presence and participation in these demonstrations.  For each demonstration below, write your prediction on this sheet before making any observations. You may be asked to send this sheet to your instructor.

 Demonstration 1: A ball is dropped to the ground.  On the diagram to the right, draw the force(s) acting on the ball as it is falling. Is there a net force acting on the ball as it falls?   Is the momentum of the ball conserved (constant) as it falls?     If you think momentum is conserved, state why.  If not, describe a system, that includes the ball, in which the total momentum is conserved.     Only after you have made your predictions click here to observe the results. Compare to your predictions and explain any differences. (The ball is in the air, falling) Demonstration 2 A ball is dropped to the ground. The ball hits the ground and bounces up. It has the same speed just before and just after it hits the ground.  On the diagram to the right, draw the force(s) acting on the ball while it is in contact with the ground. Is there a net force acting on the ball when it is in contact with the ground?   Is the momentum of the ball conserved (constant) from just before it hits the ground until just after it leaves the ground?   If you think momentum is conserved, state why.  If not, describe a system, that includes the ball, in which the total momentum is conserved.     Only after you have made your predictions click here to observe the results. Compare to your predictions and explain any differences. (The ball is in contact with the ground, bouncing.) Demonstration 3: Two objects A and B, of equal mass, collide and bounce apart. One of the objects is initially traveling to the right, and the other object is initially at rest. In the lower half of the box on the right, draw the force(s) acting on each object as they collide (while they are in contact with each other). How do the magnitudes of these forces compare? Compare the momentum of either object just before the collision to its momentum just after the collision.   Compare the total (combined) momentum of the two objects before the collision to their total momentum after the collision.   Only after you have made your predictions, click here to view the forces. Then click here to see a video of the collision, including measurements of the velocities.   Compare the results to your predictions. Explain any differences.     Was momentum conserved? Explain.  Demonstration 4: Two objects A and B, of equal mass, collide and stick together. One of the objects is initially traveling to the right, and the other object is initially at rest. In the lower half of the box on the right, draw the force(s) acting on each object as they collide (while they are in contact with each other). How do the magnitudes of these forces compare? Compare the momentum of either object just before the collision to its momentum just after the collision.   Compare the total (combined) momentum of the two objects before the collision to their total momentum after the collision.   Only after you have made your predictions, click here to view the forces. Then click here to see a video of the collision, including measurements of the velocities.   Compare the results to your predictions. Explain any differences.     Was momentum conserved? Explain.  Demonstration 5: A small object collides with a much more massive object initially at rest and bounces off of it. Assume that friction is so small that it may be neglected. In the box to the right, draw the force(s) acting on each object as they collide (while they are in contact with each other). How do the magnitudes of these forces compare?   Is the momentum of the small object conserved?   Is the momentum of the large object conserved?   Is the combined momentum of the two objects conserved?   Only after you have made your predictions, click here to view the forces.   Compare your results to your predictions. Explain any differences. 