AP Physics 1 Question 100: Answer and Explanation
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Question below refers to the following information:
Block B is at rest on a smooth tabletop. It is attached to a long spring, which in turn is anchored to the wall. Identical block A slides toward and collides with block B. Consider two collisions, each of which occupies a duration of about 0.10 s:
Collision I: Block A bounces back off of block B.
Collision II: Block A sticks to block B.
In which collision, if either, does block B move faster immediately after the collision?
- A. In collision I, because block A experiences a larger change in momentum, and conservation of momentum requires that block B does as well.
- B. In collision I, because block A experiences a larger change in kinetic energy, and conservation of energy requires that block B does as well.
- C. In neither collision, because conservation of momentum requires that both blocks must have the same momentum as each other in each collision.
- D. In neither collision, because conservation of momentum requires that both blocks must change their momentum by the same amount in each collision.
Correct Answer: A
A-There's no indication that energy must be conserved in collision 1. However, momentum is always conserved in a collision. When block A bounces, its momentum has to change to zero and then change even more to go back the other way. Since block A changes momentum by more in collision I, block B must as well because conservation means that any momentum change by block A must be picked up by block B. Choices C and D are wrong because, among other things, they use conservation of momentum to draw conclusions about two separate collisions; momentum conservation means that total momentum remains the same before and after a single collision, not in all possible collisions.