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A 1 kg ball moving downward at 10 m/s strikes a heavy platform that is moving upward at 5 m/s; the collision is elastic. Find the magnitude of the impulse delivered by the platform to the ball.

1. 15 N-s
2. 10 N-s
3. 20 N-s
4. 30 N-s

Correct answer: 30 N-s

Solution

Relative to the platform (moving up at 5) the ball approaches at 15 m/s; after elastic rebound it leaves at 15 m/s upward relative to platform, i.e. 20 m/s up in ground frame; impulse = m*delta v = 1*(20 - (-10)) = 30 N-s.

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