AP Physics 1 Question 289: Answer and Explanation
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7. Two balls collide as shown above. Given the final direction of the second ball's motion after the collision, which of the following is a possible direction for the first ball to move after the collision?
Correct Answer: D
In any collision, the total momentum of the system must be conserved. Prior to the collision, you see that all of the momentum is horizontal. Therefore, the net momentum after the collision must also be purely horizontal. If one ball is moving up after the collision, the other must be moving down to cancel out that vertical momentum.