The bob of a simple pendulum is a hollow sphere filled with water. How will the period of oscillation change, if the water begins to drain out of the hollow sphere?

dear donga,

                   If water begins to drain out of the hollow sphere, its centre of mass begins to shift below the centre of the sphere. Hence the length of the pendulum increases and time period also increases. When the entire water is drained out of the sphere, the centre of mass shifts to centre of sphere and the time period T attains its initial value.