A) The direction of motion of the ball changes every 10 seconds = correct
b) speed of the ball changes evrey 10 seconds: 11 m/s to 9 m/s to 11 m/s...
9 m/s in the direction opposite to the speed of the train.
c) average speed: (11*10+9*10) / 20 = 10 m/sec
this remains a constant for every 20 sec
d) acceleration of the ball = 0 and acceleration of train = 0
the acceleration is 0 whether from the train or from the ground, except when the ball bounces at the walls.
acceleration from the ground:
when the ball bounces at the forward wall:
velocity changes from 11 m/s to -9 m/sec. So - 20 / T m/sec/sec
where T = time duration of the collision of the ball with the wall.
velocity changes from - 9 m/sec to 11 m/sec: so 20/T m/sec/sec
acceleration as seen from the train:
at the forward wall: 1 m/s to -1 m/s : - 2/T m/sec/sec
at the rear wall: -1 m/s to 1 m/s : 2 /T m/sec/sec
So the accelerations are different during the collisions. But in between the collisions, the ball travels with uniform speed, so acceleration is 0. from both reference frames.