When a cricket ball is thrown vertically upwards, it reaches a maximum height of 5 metres.
(a) What was the initial speed of the ball?
(b) How much time is taken by the ball to reach the highest point? $(g=10\ ms^{-2})$

Given: Final velocity of the cricket ball $v=0$, maximum height $h=5\ m$ and gravitational acceleration $g=10\ m/s^2$

To do: (a). To find the initial speed of the ball.

(b). To find the time taken by the ball to reach the highest point.

Solution: 

(a). Let $u$ be the initial velocity of the cricket ball.

On using the second equation of motion $v^2=u^2+2gh$

$0=u^2-2\times10\times5$     [Here $g$ is negative]

Or $0=u^2-100$

Or $u^2=100$

or $u=10\ m/s$

Therefore, the initial velocity of the cricket ball is $10\ m/s$.

(b). Let $t$ be the time taken by the ball to reach the highest point.

On using the equation of motion, $v=u+gt$

$0=10+(-10)t$

Or $10t=10$

Or $t=\frac{10}{10}$

Or $t=1\ s$

Therefore, the time taken by the ball to reach the highest point is $1\ s$.