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Two persons of masses 55 kg and 65 kg respectively, are at the opposite ends of a boat. The length of the boat is 3.0 m and weighs 100 kg. The 55 kg man walks up to the 65 kg man and sits with him. If the boat is in still water the centre of mass of the system shifts by:

1. 3.0 m
2. 2.3 m
3. zero
4. 0.75 m

Correct answer: zero

Solution

The center of mass of the system (boat + two persons) does not shift because there are no external forces acting on the system in the horizontal direction. The internal movement of the 55 kg man is balanced by the boat's movement in the opposite direction to conserve the center of mass.

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