Question

A body of mass $10\text{kg}$ dropped from a height $20\text{m}$, acquires a velocity of $10\text{m/s}$ when it reaches ground. Magnitude of work done by air resistance (in $\text{J}$) is

Answer 1

Applying work energy theorem,
$W_{\text{ext}}=\Delta K.E$
$\Rightarrow W_{\text{gravity}}+W_{\text{air resistance}}=K.E_f-K.E_i$
$\Rightarrow mgh+W_{\text{air resistance}}=\frac{1}{2}m{v}^2-\frac{1}{2}m{u}^2$
$\Rightarrow (10\times 10\times 20)+W_{\text{air resistance}}=\frac{1}{2}\times 10\times {10}^2-0$
$[m=10\text{kg},h=20\text{m},v=10\text{m/s},u=0$
$\Rightarrow 2000+W_{\text{air resistance}}=500$
$\Rightarrow W_{\text{air resistance}}=-1500\text{J}$
Magnitude of Work done by air resistance is $1500\text{J}$