question_answer 39)

                  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 directions of motion of the ball. As seen from the ground.                 (a) the direction of motion of the ball changes every 10 seconds.                              (b) speed of ball changes every 10 seconds,                 (c) average speed of ball over any 20 second interval is fixed.                 (d) the acceleration of ball is the same as from the train.                

Answer:

                  (b, c, d) (a) As the ball is in the train, its direction of motion is the same as that of the train and does not change w.r.t. the observer on the ground. (b) Time taken by the ball to travel a distance of 10 m is (10 m) / (1 m/s) = 10 s. Since ball comes to rest (though temporarily) after each collision, speed of the ball changes after every 10 s. (c) Since the ball is bouncing elastically between the walls, its KE is conserved over any 20 s interval (10 s before collision and 10 s after collision). A such, its average speed over the interval is constant. (d) Since the train moving with a constant velocity of 10 m/s is an inertial reference frame, the acceleration of the ball is the same as seen from the train.