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
• B. 12.5 m/s
• C. 15 m/s
• D. 17.5 m/s

Answer 1

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,
$0=25kg\times 5m/s-10kg\times v\Rightarrow v=12.5m/s$
Hence, the correct option is (b).