A man weighing `80 kg` is standing on a trolley weighting `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 :

Explanation

Momentum is conserved. System consists of man and trolley.
Momentum of system = Momentum of the man
`(320 + 80) y=80 xx 1=v=0.2 ms^(-1)`
Displacement of man = 4 x 1 = 4 m
Displacement of system `= (0.2) xx 4 = 0.8 m `
` therefore ` Displacement of man relative to ground
`=(4-0.8 )= 3.2 m`