Question

Two balls of the same size but of different material, 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?
A- Iron ball is faster than the rubber
B- rubber ball is faster than iron ball
C- both moves at the same speed
D- none of the above

Answer 1

initially both the balls one made of iron and the other made of rubber are kept on the floor of the moving train. The floor is frictionless and thus, there will be no frictional force. The train suddenly stops, so we can use the concept of inertia and Newton’s first law.

Step by step answer: Yes. the balls will start rolling in the direction in which the train was moving. This happens because of the inertia. - Inertia is defined as the inherent property of the body which opposes any change in its state of rest or motion. Inertia wants if a body is at rest, it should remain at rest. If a body is in motion it should remain in motion
Since the masses of the balls are not the same, therefore, forces are not same on both the balls. Thus, the balls will move with different speeds. The inertia of an iron ball is greater than a rubber ball because it is more massive than the rubber ball. Thus, the rubber ball will move fast.

Hence, the correct option is (B).

Note: In the context of Newton's Second Law, mass is a measure of the inertia of an object. Greater be the mass greater is its inertia or we can say greater is the inertia greater is the mass. Inertia and mass have some relationship. So, both the balls will start moving and the inertia is dependent on the mass of the object.