Question

A man weighing 80 kg is standing on a trolley weighing 320 kg. The trolley is resting on frictionless horizontal rails. If the man starts walking on the trolley along the rails at a speed of 1 m/s relative to trolley, then after 4 s, his displacement relative to the ground will be:

• A. 4.2 m
• B. 4.8 m
• C. 3.2 m
• D. 3.0 m

Answer 1

The correct option is C 3.2 m

To conserve the momentum, trolley will move in the direction opposite to man's movement.

let trolleys velocity is v and its mass is m. Let man's velocity is $V$ and his mass is $M$, then we have

$m\times v=-(M+m)\times V..................\left(1\right)$

we have $m=320kg,M=80kg,V=1m/s$

by substituting this values in $\left(1\right)$, we get $v=-0.2m/s.$

Since man has moved for 4 s, distance moved by trolley $-0.2\times 4=-0.8$ m

man has moved $4$ m but trolley has moved $1$ m in opposite direction.

hence displacement of man with respect to ground is $4-0.8=3.2$ m