Question

Inside the railway car a plumb bob (a heavy mass suspended by a thread) is suspended from the roof and a helium filled balloon is tied by a string to the floor of the car. When the railway car accelerates to the right, then
A. Both the plumb bob and balloon move to the left
B. Both the plumb bob and balloon moves to the right
C. Plumb bob moves to the left and the balloon moves to the right
D. Plumb bob moves to the right and the balloon moves to the left

Answer 1

To answer this problem, consider the inertia of plumb bob. So, when the railway car accelerates to the right, Bob tries to maintain its original state by moving in the opposite direction of the railway car. While, the balloon moves to the same direction in which the car accelerates. This is due to the increase in the pressure at one end. Using these basics, the direction of the plumb bob as well as the balloon can be found when the railway car accelerates to the right.

Complete answer:
When the railway car accelerates to the right, the plumb bob due to its inertia tries to be in its original state or position. To be in its original state, it moves to the left when the railway car moves to the right.
But when we consider balloons, its case is different compared to plumb bob. As it is lighter than air, it floats in air. The air tries to be in its initial state due to its inertia. So, when a railway car accelerates to the right, air moves to the left. This increases the pressure at the left and hence the balloon moves to the right.
Hence, when the railway car accelerates to the right then plumb bob moves to the left and the balloon moves to the right.

So, the correct answer is “Option C”.

Note:
Students should know that a body moving in a certain direction cannot change its direction or state unless a force is applied on it. There are two terms related to know i.e. Inertia of motion and Inertia of direction. Inertia of motion is the inherent property of a body by virtue of which it cannot change its state of motion. While, the inertia of direction is the inherent property of the body by virtue of which it cannot change its direction.