Question

A tunnel has been dug through the centre of the earth and a ball is released in it. It will reach the other end of the tunnel after

• A. 84.6 minutes
• B. 42.3 minutes
• C. 1 day
• D. Will not reach the other end

Answer 1

Answer: (B)

42.3 minutes

Answer 2

Ball execute S.H.M. inside the tunnel with time period $T = 2\pi\sqrt{\frac{R}{g}} = 84.63\mspace{6mu}\min$

Hence time to reach the ball from one end to the other end of the tunnel $t = \frac{84.63}{2} = 42.3\mspace{6mu} min.$