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  4. Assertion: The bob of a simple pendulum is a ball full of water, if a fine hole is made in the bottom of the ball, the time period first increases and then decreases. Reason: As water flows out of the bob the weight of bob decreases.

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Q. Assertion: The bob of a simple pendulum is a ball full of water, if a fine hole is made in the bottom of the ball, the time period first increases and then decreases.
Reason: As water flows out of the bob the weight of bob decreases.

AIIMSAIIMS 2009Oscillations

Solution:

When ball is completely filled with water, the centre of gravity of the pendulum is at the centre of the ball when water starts flowing out, the centre of gravity shifts below, thus increasing the length of pendulum, and increasing time period. When ball is more than half empty the centre of gravity again rises up so length of pendulum decreases and time period also decreases.