#### A ball is bouncing elastically with a speed 1 m/s between walls of a railway compartment of size 10 m in a direction perpendicular to walls. The train is moving at a constant velocity of 10 m/s parallel to the direction of motion of the ball. As seen from the ground,a) the direction of motion of the ball changes every 10 secondsb) speed of ball changes every 10 secondsc) average speed of ball over any 20 seconds intervals is fixedd) the acceleration of ball is the same as from the train

The correct answer is the option:

(b) The speed of the ball changes every 10 seconds.

(c) average speed of the ball over any 20 seconds interval is fixed.

(d) the acceleration of the ball is the same as from the train.

Explanation: If we observe the motion from the ground, we will see that the ball strikes with the wall after every 10 seconds. The direction of the ball is the same since it is moving at a very small speed in the moving train, therefore, it will not change w.r.t. observer from the earth.

The speed of ball can change after a collision, hence, option (a) will be discarded and opt (b) is verified.

Average speed of the ball at any time remain same or is 1 m/s, i.e., it is uniform.

Hence opt (c) is also verified.

When the ball strikes to the wall, initial speed of the ball will be in the direction of the moving train w.r.t. the ground as well as its speed will also change (vTG)

Thus,

The speed of the ball after collision with a side of the train is in the opposite direction of the train

Thus, the magnitude of acceleration on both the walls of the compartment will be the same, but in opposite direction. Hence, opt (b), (c) & (d) are verified here.