A metal ball is suspended from the roof of a train by a string. When the train is in uniform motion, will the string remain vertical? What happens to the string if the train accelerates?

Explanation

When the train moves in a straight line with a uniform speed, the speeds of the string and the ball are the same as that of the train. Hence the string remains vertical. When the train accelerates, the speed of the attached end of the string increases but the metal ball, because of its inertia of rest, tends to resist the change. As a result, the string shifts obliquely backwards with respect to the vertical, i.e., shifts opposite to the direction of acceleration.