A man weighing  is standing in a trolley weighing . The trolley is resting on frictionless horizontal rails. If the man starts walking on the trolley with a speed of , then after  his displacement relative to the ground will be in m is

Displacement of the man with respect to trolley in

Position of centre of mass remain constant

Alternatively: If the man starts walking on the trolley in the forward direction then whole system will move in backward direction with same momentum.

Momentum of man in forward direction = Momentum of system (man + trolley) in backward direction

So the velocity of man w.r.t. ground

 Displacement of man w.r.t. ground