A boy is running to the east and jumps into a stationary boat in a lake. Once the boy lands in the boat, the boat moves to the east. Which best compares the initial momentum of the boy with the final momentum of the boy/boat system?

The boat is initially stationary, so the system’s initial momentum is just the boy’s momentum, m_boy v_boy. Assuming there is no significant external horizontal force, momentum is conserved when he lands, so the boy-and-boat system’s final momentum equals that initial momentum. The combined system moves more slowly because the boat adds mass, which is consistent with choice A, “the initial momentum of the boy is equal to the boy/boat.”

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