Question

A ball is dropped from a bridge of $122.5$ metre above a river. After the ball has been falling for two seconds, a second ball is thrown straight down after it. Initial velocity of second ball so that both hit the water at the same time is

• A. 49 m/s
• B. 55.5 m/s
• C. 26.1 m/s
• D. 9.8 m/s

Answer 1

Answer: (C)

$-h=-\frac{1}{2}g{t}^2{1}^{\text{st }}\text{ ball }$
$\Rightarrow 122.5=\frac{1}{2}\times 9.8t^2$
$\to t^2=25\Rightarrow t=5s$
Another ball is dropped after $2$ second so it took only $(5-2)=3s$
$-122.5=-u(3)-\frac{1}{2}\times 9.8\times 3^2$
$\Rightarrow 122.5=3u+4.9\times 9$
$\Rightarrow 3u=78.4$
$\Rightarrow u=26.1s$