Question

A passenger in a train hangs a stone from a string and holds it vertically. It is observed that the string is vertical as seen from the man.

Which of the following could be true about the motion of the train?

I. The train is at rest.

II. The train is moving with a constant velocity.
III. The train is increasing its speed.
IV. The train is decreasing its speed.

• A. I only
• B. III only
• C. I or II, but not III or IV
• D. III or IV, but not I or II
• E. IV only

Answer 1

The correct option is C I or II, but not III or IV

When the train is either at rest or moving with constant velocity, then the acceleration of the train is zero and hence there is no pseudo force acting on the stone in the horizontal direction which makes the string tied to the stone to stay in vertical direction.

If the train either increases or decreases its speed, then the stone experiences a pseudo force in the horizontal direction due the acceleration of train and thus the string gets tilted by some angle with the vertical direction.

Thus option C is correct.