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Question

# A boy of mass 25 kg stands on a board of mass 10 kg which in turn is kept on a frictionless horizontal surface. The boy makes a jump with a velocity 5 m/s in the horizontal direction with respect to the board. With what velocity does the board recoil?

A

10 m/s

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B

12.5 m/s

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C

15 m/s

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D

17.5 m/s

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Solution

## The correct option is B 12.5 m/s Considering the board+boy as a system, there is no external force in the horizontal direction. Hence, momentum will remain conserved in the horizontal direction. If the board recoils at a speed v, 0=25 kg×5 m/s−10 kg×v⇒v=12.5 m/s Hence, the correct option is (b).

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