A ball falls from height h. After 1 second, another ball falls freely from a point 20 m below the point from where the first ball falls. Both of them reach the ground at the same time. What is the value of h

Options

A. 11.2 m
B. 21.2 m
C. 31.2 m
D. 41.2 m

Answer

C

Explanation

For 1st ball `h = 1/2 g t^2`
For 2st ball `(h-20) = 1/2 g( t-1)^2`
By (i) and (ii)
(1/2 g t^2 - 20) = 1/2 g (t^2 - 2t + 1)`
`rArr 20 = 1/2 g (-1 + 2t)`
`rArr 20 = -5 + 10t rArr t = 2.5sec`
` therefore h = 1/2 xx 10 xx (2.5)^2 = 31.2m`