SAT Subject Physics Practice Question 365: Answer and Explanation
61. A 30 kg child is standing in the back of a stationary 10 kg wagon that is free to move.The child jumps from the wagon into his mother's arms. Which of the following statements is correct at the moment he jumps from the wagon?
A. The child's velocity is greater than the wagon's velocity.
B. The wagon's velocity is greater than the child's velocity
C. The impulse exerted by the child on the wagon is the same as the momentum gained by the wagon.
D. The impulse exerted by the wagon on the child is larger than the momentum gained by the child.
E. None of these statements is correct.
Correct Answer: B
The correct answer is (B). When the child jumps into his mother's arms, he pushes on the wagon as he jumps. The momentum the child gains when he jumps is passed on to the wagon, too. They both have the same momentum, but in opposite directions.The wagon must have a greater velocity since it has less mass.