Two balls of the same size but of different materials, rubber and iron are kept on the smooth floor of a moving train. The brakes are applied suddenly to stop the train. Will the balls start rolling? If so, in which direction? Will they move with the same speed? Give reasons for your answer.

Yes the balls start rolling in the direction of train was moving.. When brakes are applied, train will come to rest and balls try to attain rest because of inertia balls remain in motion and they began to roll. Since the mass between two balls are not same inertia of iron ball is greater than the inertia of rubber ball. Hence rubber ball rolls faster than iron ball.