Question

An object dropped from a height of $20m$ is undergoing free fall. What is its velocity just before it touches the ground? (Take $g=10m{s}^{-2}$)

• A. $10m{s}^{-1}$
• B. $20m{s}^{-1}$
• C. $15m{s}^{-1}$
• D. $17m{s}^{-1}$

Answer 1

The correct option is B $20m{s}^{-1}$

Given Height $h=20m$
Acceleration due to gravity $g=10m{s}^{-2}$.
Initially the body is dropped, so initial velocity $u=0m{s}^{-1}$.
According to the conservation of energy, energy will remain constant throughout. So, potential energy at top is equal to kinetic energy at bottom
$mgh=\frac{1}{2}m{v}^2$
or $v^2=2gh=2\times 10\times 20$
or $v^2=400$
or $v=20m{s}^{-1}$