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A 60 kg man runs along a road and leaps onto a stationary trolley of mass 120 kg. Immediately afterward the trolley (with the man on board) moves at 2 m/s. What was the man's running speed (in m/s) just before he jumped on?

1. 4
2. 6
3. 2
4. 3

Correct answer: 6

Solution

Conserving momentum: the man's forward momentum equals the combined momentum of man plus trolley after he lands, giving v = 6 m/s.

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