Since, no external force is acting on the system, we can apply conservation of linear momentum. Speed of 200kg trolley =
36×1000
60×60
=10ms−1 If u be the initial velocity of trolley, vb be the absolute velocity of the boy after the beginning of journey of the boy, their relative velocity is 4. So,v′−vb=4⇒vb=(v′−4) Now, applying law of conservation of momentum, Momentum before the boy begins to run = Momentum after the beginning of boy's running ⇒220×10=200v′+20(v′−4) 2200=220v′−80⇒220v′=2280 v′=