Question

When a ball is thrown inside a moving bus, does its kinetic energy depend on the speed of the bus? Explain.

Answer 1

Yes, the kinetic energy of the ball depends upon the speed of the bus. When we are sitting on a bus, we are actually moving along with the bus with a speed equal to the speed of the bus. So, if we are holding a ball, that already means that the ball also has the same speed as ours or the same as the bus. This simply means that the ball and we both are a part of the system, which is moving at the same speed as the speed of the bus. So, if the speed of the bus changes then the speed of the ball changes too.

When we throw a ball inside the bus with some speed, then we are just going to provide some extra speed to the ball along with the speed of the bus. But this can only be observed by a person standing outside the bus, i.e. a stationary observer (with respect to the earth). If the observer would be inside the bus, then he would also become a part of the system which is moving with a speed same as the speed of the bus, then the observer will observe the ball with respect to him.

Hence, we can say that the kinetic energy of the ball depends on the speed of the bus but only if we are a stationary observer (with respect to earth)