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A boat of length 10 m and mass 450 kg floats at rest in still water. A man of mass 50 kg stands at one end and walks to the other end, then stops. Find the magnitude of displacement of the boat relative to the ground.

1. Zero
2. 1 m
3. 2 m
4. 5 m

Correct answer: 1 m

Solution

No external horizontal force => CM of system stays fixed. Let boat shift by d in the direction opposite to man's walk. Man's displacement relative to ground = (10 - d) in the direction of walk. CM condition: M_boat * d = M_man * (10 - d). 450*d = 50*(10-d). 450d = 500 - 50d. 500d = 500. d = 1 m.

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