question_answer

A man weighing 80 kg is standing in a trolley weighing 320 kg. The trolley is resting on frictionless horizontal rails. If the man starts walking on the trolley with a speed of 1 m / s, then after 4 sec his displacement relative to the ground will be              [CPMT 1988, 89, 2002]

A)                         5 m                 

B)                         4.8 m            

C)                         3.2 m              

D)                         3.0 m

Correct Answer: C

Solution :

                            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 Þ \[80\times 1=(80+320)\times v\]Þ \[v=0.2\ m/s\] So the velocity of man w.r.t. ground \[1.0-0.2=0.8\ m/s\]             \[\therefore \] Displacement of man w.r.t. ground \[=0.8\times 4=3.2\ m\]