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An open watertight railway wagon of mass 5 x 10³ kg coasts frictionlessly on a track at an initial speed of 1.2 m/s. Rain falls vertically into the wagon. What is the wagon's speed after it has collected 10³ kg of rainwater?

1. 0.5 m/s
2. 2 m/s
3. 1 m/s
4. 1.5 m/s

Correct answer: 1 m/s

Solution

Horizontal momentum is conserved since rain has no horizontal velocity: 5000*1.2 = 6000*v, giving v = 1 m/s.

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