A stone is dropped from a height of 20 m.
(i) How long will it take to reach the ground?
(ii) What will be its speed when it hits the ground? $(g=10\ m/s^2)$

Given: Initial velocity $u=0$

Height $h=20\ m$

Gravitational acceleration $g=10\ m/s^2$

To do: (i). To find how long will it take to reach the ground.

(ii). To find its speed when it hits the ground.

Solution: Let $v$ be the final velocity when it hits the ground and $t$ be the time taken to reach the ground.

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

$v^2=0+2\times10\times20$

Or $v^2=400$

Or $v=\sqrt{400}$

Or $v=20\ m/s$

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

$20=0+10\times t$

Or $10t=20$

Or $t=\frac{20}{10}$

Or $t=2\ s$

Therefore, it will take $2\ s$ to reach the ground and its speed when it hits the ground is $20\ m/s$.

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Updated on: 10-Oct-2022

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