A ball is thrown up and attains a maximum height of 20 m. Calculate its initial speed. $(Take the value of \text{g as }10{\text{m/s}}^2)$

Given:
Max height = 20 m
$\text{g}=10{\text{m/s}}^{\text{2}}$
We know that when a ball is thrown in the air, its speed goes on reducing as it goes up. This happens due to the gravitation force that is opposing the upward motion of the ball. At the maximum height, the upward speed becomes zero before the ball starts falling down.
Therefore, we can say:
Final velocity, V = 0

To find: 
Initial velocity

Solution:
We can use the following kinematical equation to find the initial velocity:
${\text{v}}^{\text{2}}-{\text{u}}^{\text{2}}=2\text{gh}$

${\text{o}}^2-{\text{u}}^{\text{2}}=\left(2\right)\left(-10\right)\left(20\right)$

$-{\text{u}}^{\text{2}}=-400$

$\text{u}=20{\text{ms}}^{-1}$