Question

A boy of mass 50kg is standing on a frictionless surface. He throws a ball of mass 2kg away from him with a speed of 10m/s. Find the final speed of the centre of mass.

(a) 0m/s

(b) 20m/s

(c) 10m/s

(d) 0.4m/s

The question was asked in exam.

Question is from System of Particles topic in section System of Particles and Rotational Motion of Physics – Class 11

Answer 1

Correct choice is (a) 0m/s

The best explanation: Initially the boy was at rest and there is no friction, so the net force on the ball-boy system is zero.                                                                                                                                                                                 Hence, their momentum change will be zero and final velocity of centre will also be zero.                                     Note that boy will move backwards with a speed of 2*10/50 = 0.4m/s.