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.

Dear Student,
Given,
Two balls of same size but are of two different materials, rubber and iron. Since, the density of iron is much higher than rubber, iron ball will have a greater mass than the rubber ball.
Suppose, the train is moving along +x - direction. When the brakes are applied, the train starts decelerating along +x-direction. We can say that there acts an acceleration on the train in -x - direction.
While braking the train acts as a non-inertial frame. 
So, in the case of braking, the balls lying on the smooth (frictionless) floor of the train will experience a pseudo force along the +x-direction.
Since, the mass of the iron ball is more, so, due to inertia, iron ball will take up less speed than the rubber ball.

Regards

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