SHM

See , the time period of a physical pendulum = 2 π ( Im g d ) ^{1 / 2}

where : -

I = Moment of inertia of the rigid body about the suspension point .

d = Distance from the center of mass of the rigid body to the suspension point .

Now , when the bob is fully filled with water , it behaves like a " solid " sphere , not a hollow one . But , slowly as water drains out , it gradually becomes hollow , thereby changing its time period .

Now , try yourself in proving that , initial time period of the pendulum : -

T₁ = 2 Ï€ ( 7 d5 g ) ^{1 / 2}

And , the final time period : -

T₂ = 2 Ï€ ( 5 d3 g ) ^{1 / 2}

N . B . : - In between these two phenomena , if you want to calculate the time period , then you have to use integration to find out the moment of inertia for a general level of water in the bob . It would then be quite difficult , but certainly not impossible .

Ah , I misinterpreted the question . The answer given by Abhisek is correct . Abhisek , you should note that the time period will not be the same anymore , but will be another constant .

The time period of the filled bob will differ from that of the empty bob ?
As far as i know time period is independent of the bob's mass... [7]

That would be true if the bob is assumed to be massless : )

See , if the bob has some mass , would it form a simple pendulum any more ? It would be a physical pendulum , won ' t it ?

thnks