Question

A bob of pendulum was filled with $Hg$ and entire $Hg$ is drained out, then the time period of pendulum

• A. remains unchanged
• B. decreases
• C. increases
• D. increases then decreases

Answer 1

Answer: (D)

When a hole is made in the bottom of a bob filled with mercury, the mercury flows out slowly through the hole.
As the mercury flows out of the bob, the centre of mass of the system will first move down and then will come up.
Due to this the equivalent length of pendulum $L$ and hence time period $T$ first increases, reaches a maximum value and then decreases till it becomes equal to its initial value.