F.4 phys,,Explain the motion of the ball in terms of inertia

There is an external force acting on the train, so the train is moving with acceleration.

For the ball, due to inertia, the ball will stay on its initial position, governed by Newton's 1st law of motion.

However, the observer is on the train which himself is accelerating, and the ball stays on its initial position.

As a result, the boy will see the ball rolls in a direction opposite to the motion of the train.