## 21 May 2009

### Conceptual physics exam review – momentum

A few weeks ago I described a review exercise for the AP exam involving a classroom response system, aka “clickers.” I divide the class randomly into teams of two, and ask a series of multiple choice questions. Teams are given plenty of time to discuss the answer, we go over the answer, and points are awarded – one point for the correct answer, and one bonus point for any group that got the answer WRONG. (Why this scoring? It encourages discussion within the group, but discourages bright students from giving away the answer to the class.)

This morning I covered a colleague’s 9th grade conceptual physics class. John asked me to run a review on the concepts of impulse, momentum, and conservation of momentum. He warned me that the students were likely rusty… it’s been months since they covered these ideas, and the students weren’t necessarily nailing everything to begin with.

In such a case, I like to ask a variety of multiple choice questions about the same physical situation. Many of the questions are simple recall, intended to remind everyone about a fundamental fact that must be used with a following question. (In the exercise, I reveal and discuss each question one at a time.) Because I don’t change the problem setup, students don’t have to read and refocus for each question – they can focus directly on the relevant concepts. The problems below are inspired by an AP free response problem. I distributed JUST the figure and the three-sentence description in hard copy to each group.

Beware of your time constraints. I tend to allow 1.0-1.5 minutes for each question, followed by several minutes of discussion. These take a long time! I got through seven in 30 minutes.

A 70 kg woman and her 35 kg son are standing at rest on an ice rink as shown above. They push against each other for a time of 0.60 s, causing them to glide apart. The speed of the woman immediately after they separate is 0.55 m/s.

1. What is the equation for momentum?
(A) mv
(B) ½mv2
(C) Fv
(D) mg
(E) ma

2. What are the units of momentum?
(A) newtons
(B) m/s2
(C) joules
(D) watts
(E) kg·m/s

3. What is the magnitude of the total momentum of the two people before they push?
(A) 39 kg·m/s
(B) 19 kg·m/s
(C) 58 kg·m/s
(D) 20 kg·m/s
(E) 0 kg·m/s

What is the magnitude of the total momentum of the two people AFTER they push?
(A) 39 kg·m/s
(B) 19 kg·m/s
(C) 58 kg·m/s
(D) 20 kg·m/s
(E) 0 kg·m/s

5. What is the woman’s momentum after the push?
(A) 39 kg·m/s to the left
(B) 19 kg·m/s to the left
(C) 58 kg·m/s to the left
(D) 20 kg·m/s to the left
(E) 0 kg·m/s to the left

6. Who has more momentum after the push?
(A) The woman
(B) Her son
(C) Both have the same momentum

7. What is her son’s momentum after the push?
(A) 39 kg·m/s to the right
(B) 19 kg·m/s to the right
(C) 58 kg·m/s to the right
(D) 20 kg·m/s to the right
(E) 0 kg·m/s to the right

8. Who moves faster after the push?
(A) The woman
(B) Her son
(C) Both move the same speed

9. What is her son’s speed after the push?
(A) 70 m/s
(B) 0.23 m/s
(C) 0.90 m/s
(D) 0.55 m/s
(E) 1.1 m/s

10. Whose momentum changes more during the push?
(A) The woman
(B) Her son
(C) Both have the same momentum change

11. What is the term that means “change in momentum?”
(A) Kinetic energy
(B) force
(C) impulse
(D) potential energy
(E) power

12. Impulse is change in momentum. What is another equation for impulse?
(A) Mass time acceleration
(B) Mass times velocity
(C) Force times velocity
(D) Force times time
(E) Mass times gravitational field

13. Who experiences more force during the push?
(A) The woman
(B) Her son
(C) Both experience the same force

14. Who experiences more acceleration during the push?
(A) The woman
(B) Her son
(C) Both experience the same acceleration

15. By how much does the woman’s momentum change during the push?
(A) 39 kg·m/s
(B) 19 kg·m/s
(C) 58 kg·m/s
(D) 20 kg·m/s
(E) 0 kg·m/s

16. What are the units of force?
(A) newtons
(B) m/s2
(C) joules
(D) watts
(E) kg·m/s

17. How much force did the woman experience during the push?
(A) 21 N
(B) 71 N
(C) 23 N
(D) 700 N
(E) 65 N