Suppose the bob in a simple pendulum is a hollow sphere with a small hole at the bottom. The sphere is filled with water and is made to oscillate. What happens to the period ?

Explanation

T depends on length of the simple pendulum which is the distance between centre of gravity of the bob and the point of suspension. As the water flows out the the C.G. shifts downwards. So the effective length increases and period increases. When the whole water has flown out the C.G again coincides with the centre of the bob and the time period becomes same as in the beginning. Thus the period first increases and then decreases to the original value.