A simple pendulum of length 40 cm is taken inside a deep mine. Assume for the time being that the mine is 1600 km deep. Calculate the time period of the pendulum there. Radius of the earth = 6400 km.
1.47 s
given length 
Let M be the mass of earth
--------->(1)
Gravity at depth ‘d’ will be
[Let ‘r’ be the distance from center of the earth to that point
]
---------->(2)
From (1) & (2)
[‘g’ on surface is 
]
Time period at that depth 
= 1.47 sec
1