A boy of mass 50 kg produces an acceleration of `2(m)/(s^2)` in a block of mass 20 kg by pushing it in horizontal direction. The boy moves with the block such that boy and the block have same acceleration. There is no friction between the block and fixed horizontal surface but there is friction between foot of the boy and horizontal surface. Find friction force (in Newton) exerted by the horizontal surface on the boy.

Options

A. 120
B. 140
C. 160
D. 200

Answer

B

Explanation

For Block `F=20.2=40N`. As boy exerts 40 N force on block, block exerts 40 N force on the boy, in opposite direction. As boy is also moving with same acceleration `-40=50.2`
`f=140N`
Aliter: Consider the boy`+`block system. The only external force is friction acting on boy `f`
`f=(M_(boy)+M_(block))a=140N`