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 speed `1 m//s` (w.r.t. to trolley) then after `4 s` his displacement relative to the ground will be :

Options

A. 5 m
B. 4.8 m
C. 3.2 m
D. 3.0 m

Answer

C

Explanation

( c) As no external force acting on the system, the displacement of centre of mass of the system should be zero. Let displacement of the trolley on opposite direction of motion of the man is `x`.
Hence `Delat x_(cm) = 0 = (m Delta x_(man) + M Delta x_("trolley"))/(m + M)`
`80(1 xx 4 - x)+ 320(- x) = 0, x = (80 xx 4)/((320 + 80)) = 3.2 m`.