The ball is dropped from a bridge ` 122.5 m` above a river, After the ball has been falling for 2 s, a second ball is thrown straight down after it. What must its initial velocity be so that both hit the water at the same time ?

Options

A. (a) ` 40 m//s`
B. (b) ` 55.5 m//s`
C. (c ) ` 26. 1 m//s`
D. (d) ` 9. 6 m//s`

Answer

C

Explanation

Let the first ball hit the qater in `t` seconds
Using , ` S= ut + 1/2 at^2`
For first ball,
` 122.5 =0 + 1/2 xx 9.8 xx t^2 = 4.9 T^2` ltbRgt or ` t= sqrt (122.5)/(4.9) = 5 s` ltbRgt For second ball,
` 122.5 = u (5 -2) + 1/2 xx 9.8 xx (5 - 2)^2`
`= 2 u + 44 .1`
` 3 u = 122.5 + 44 .1`
` 3 u = 122.5 - 44.1 = 78.4 or u=26.1 ms^(-1)`.