A ball is hanging from a long string that is tied to the ceiling of a train car traveling eastward on horizontal tracks. An observer inside the train car sees the ball hang motionless. Draw a clearly labeled free-body diagram for the ball if (a) the train has a uniform velocity and (b) the train is speeding up uniformly. Is the net force on the ball zero in either case? Explain.

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.

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