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

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 the same momentum. Momentum of man in forward direction = Momentum of system (man + trolley) in backward direction \[\Rightarrow \]\[80\times 1=(80+320)\times v\Rightarrow v=0.2\,m/s\] 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