Question

A body of mass m dropped from a height h reaches the ground with a speed of $1.4\sqrt{gh}$. The work done by air drag is:

• A. $0.2mgh$
• B. $0.02mgh$
• C. $0.04mgh$
• D. $mgh$

Answer 1

The correct option is B $0.02mgh$

Using work energy theorem:

$W_{gravity}+W_{airresistance}=\Delta K.E.$

$mgh+W_{airresistance}=\frac{1}{2}m{v}^2$

$mgh+W_{airresistance}=\frac{1}{2}m(1.4\sqrt{gh}{)}^2$

$mgh+W_{airresistance}=\frac{1}{2}1.96\times mgh$

$mgh+W_{airresistance}=0.98\times mgh$

$W_{airresistance}=0.98\times mgh-mgh=.-02mgh$

hence correct option is B