Question

Inside a 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 move 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

Answer: (C)

Plumb bob moves to the left and the balloon moves to the right

The bob due to its inertia tries to be in its original state. So, when the car starts, it tries to resist the motion and tries to maintain its position and so it moves to the left when the car moves to the right.
But the case of the balloon is different. Balloon, being lighter than air, floats in the air. So, when the car is accelerated, the air, due to its inertia moves left which increases the pressure there and hence the balloon moves at the right.