1. Momentum

2. A note on usage:
The clicker slides in this booklet are meant to be used as stimuli to encourage class discussion. They are intended for use in a class that attempts to help students develop a coherent and sophisticated understanding of scientific thinking. They are NOT intended as items to test whether students are “right or wrong” or “know” the correct answer by one-step recall if enough cues are given. This has a number of instructional implications that are reviewed in general on the next four slides. The individual slides also contain annotations discussing their intended use.

3. Usage: 1
Feedback
One of the most important values of a clicker-response system is to provide instructors with some understanding of what students are thinking.
Good clicker questions can be highly revealing (and surprising). But the critical fact is not that the students make mistakes but to use those mistakes to probe their thinking and find out why.
This raises the importance of a rich subsequent discussion well above “letting the students know what the right answer is.”

4. Usage 2: Student-student interactions
The critical value for student learning occurs in what happens after a clicker question has obtained a mixed response from the students. The standard next cue is, “Find someone who disagreed with the answer you chose and see if you can convince them.”
After a minute or two of discussion, a second click may show students having moved dramatically towards the correct answer. A brief call for who changed their answer and why can lead to a useful exchange. When they have not moved significantly, more discussion is called for.

5. Usage: 3 Incompletely specified questions
Some items have questions that are simple if idealized assumptions are made, subtler if they are not. Part of the discussion of these items are intended to include issues of modeling, idealizations, and hidden assumptions.
Questions where answers are not provided.
In these items, the intent is to have students come up with potential answers and have the instructor collect them and write them on the board.
Occasionally, especially at the beginning of a class, it may take some time before students are willing to contribute answers. It can help if you have some prepared answers ready, walk around the class, and put up the answers as if they came from the students. This can help students get more comfortable with contributing.

6. Usage: 4 Cluster questions Problem solving items
Some questions are meant to be used as part of a group of questions. In this case, resolving the answers to individual questions is better left until the entire group is completed. The value of the questions are often in the comparison of the different items and in having students think about what changes lead to what differences and why.
Problem solving items
In these items (indicated by a pencil cluster logo), the intent is to have students work together to solve some small problem. After a few minutes, ask the groups to share their answers, vote on the different answers obtained, and have a discussion.

7. Example: Recoil Do it! (object goes backwards)
When an object at rest emits a part of itself, in order to conserve momentum, it must go back in the opposite direction.
What forces are responsible for this motion?
(object goes backwards)
Do it!

8. The diagram at the right depicts the path of two colliding steel balls rolling on a table. Which set of arrows best represents the direction of the change in momentum of each ball?
From Chandralekha Singh’s energy-momentum concept inventory

9. Two fan carts are on opposite sides of a table with their fans pointed in the same direction. Cart A is twice as heavy as cart B. When the fans are on, they cause the air to exert a constant force on the cart independent of its mass. Assume friction can be neglected. The fans are set with a timer so that after they are switched on, they stay on for a fixed length of time, Δt, and then are turned off.
Just after the fans are turned off, which is true about the momenta of the two carts? (A) pA > pB (B) pA < pB (C) pA = pB

10. Two fan carts are on opposite sides of a table with their fans pointed in the same direction. Cart A is twice as heavy as cart B. When the fans are on, they cause the air to exert a constant force of the cart independent of its mass. Assume friction can be neglected. The fans are set with a timer so that after they are switched on, they stay on for a fixed length of time, Δt, and then are turned off.
Just after the fans are turned off, which is true about the velocity of the two carts? (A) vA > vB (B) vA < vB (C) vA = vB

11. The momentum of the system is zero in case (ii).
Two identical carts A and B roll down a hill and collide as shown in the figures at the right.
(i): A starts from rest. It rolls down and collides head-on with B which is initially at rest on the ground. The two carts stick together.
(ii): A and B are at rest on opposite sides of the hill. They roll down, collide head-on and stick together.
Which statement is true about the two-cart system just before the carts collide in the two cases?
The momentum of the system is zero in case (ii).
The momentum of the system is greater in case (i) than in case (ii).
The momentum of the system is greater in case (ii) than in case ii).
The momentum of the system is the same in both cases (but not 0).
More than one statement is true.
10/22/13
Physics 131

12. The momentum of the system is zero in case (ii).
Two identical carts A and B roll down a hill and collide as shown in the figures at the right.
(i): A starts from rest. It rolls down and collides head-on with B which is initially at rest on the ground. The two carts stick together.
(ii): A and B are at rest on opposite sides of the hill. They roll down, collide head-on and stick together.
Which statement is true about the two-cart system just after the carts collide and stick in the two cases?
The momentum of the system is zero in case (ii).
The momentum of the system is greater in case (i) than in case (ii).
The momentum of the system is greater in case (ii) than in case ii).
The momentum of the system is the same in both cases (but not 0).
More than one statement is true.
10/22/13
Physics 131

13. A ball on a table slides and hits a block
A ball on a table slides and hits a block. Which ball exerts the most force on the block?
(bounces back)
(bounces off)
(captured)

14. Which ball will knock the block over?
Rigid rod
Wooden block
Ball
Pivot
A superball
A clay ball of equal mass
Both
Neither

15. Suppose you are on a cart, initially at rest on a track with negligible friction. You throw balls at a partition that is rigidly mounted on the cart. The balls bounce straight back as shown in the figure.
Is the total horizontal momentum of the person, cart, partition, plus all the balls conserved in the process of throwing the balls and having them bounce off the partition?
Yes.
No.
You are not given enough information to decide.

16. Is the cart put in motion as a result?
Suppose you are on a cart, initially at rest on a track with negligible friction. You throw balls at a partition that is rigidly mounted on the cart. The balls bounce straight back as shown in the figure.
Is the cart put in motion as a result?
Yes. Towards the left
Yes. Towards the right.
No.
You are not given enough information to decide.

17. A molecular cluster at rest collides with an atom
A molecular cluster at rest collides with an atom. As a result, the atom becomes strongly bound to the cluster and an identical atom (from a different part of the molecule) gets shot off with much higher speed. What can you say about the motion of the reformed cluster after the collision?
Before
It will be stationary.
It will move to the left.
It will move to the right.
This is not really possible, despite the claim that it is.
You can’t say anything about it from the information given.
Something else.
After