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. `40 ms^(-1)`
B. `55.5 ms^(-1)`
C. `26.1 ms^(-1)`
D. `9.6 ms^(-1)`

Answer

C

Explanation

Let the ball hit water in t second.
For first ball, `s=ut+(1)/(2)at^(2)`
`122.5=0+(1)/(2)xx9.8xxt^(2)=.9at^(2)`
`t+sqrt((122.5)/(4.9))=sqrt(25)=5s`
For second ball,
`122.5=u(5-2)+(1)/(2)xx9.8(5-2)^(2)`
`3u+44.1`
`rArr 3u=122.5-44.1`
`3u=78.4`
`rArr u=26.1 ms^(-1)`