Answer to Question #144930 in Mechanics | Relativity for Mikkos Joshua Garcia

Question

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

Answer 1

(a) The train does not accelerate.

The hanging ball does not accelerate (horizontally).

This is why the only forces acting on it are its weight and the tension of the string. Both forces are vertical and of equal magnitude.

(b) The train accelerates eastward.

The ball also has contact eastward (horizontal) acceleration.

A net force must be causing this acceleration.

An eastward component of the tension provides the horizontal net force.

The ball will be displaced west of the vertical.

Answer: net force on ball in case (a) is zero, but net force on ball is not zero in case (b).