Explanation:
a) FBD of the ball when the train has a uniform velocity
The train has uniform velocity and only two force Tension of the string and weight of the ball act on the ball such that magnitude and direction are equal and opposite. And the net force of the ball is zero.
b) FBD of the ball when the train is speeding up uniformly.
The train accelerates eastward that is why the ball has an eastward acceleration. So, there must be net force in eastward direction from Newtons law. From FBD we can see that the eastward component of the Tension provides the necessary acceleration.
