Question

An object of mass 5 kg falls from rest through a vertical distance of 20 m. and attains a velocity of 10 m/s. How much work is done by the resistance of the air on the object? $(g=10m/s^2)$

• A. $-550J$
• B. $-750J$
• C. $-850J$
• D. $-450J$

Answer 1

The correct option is B $-750J$

Applying work-energy theorem, work done by all the forces=change in kinetic energy
or $W_{mg}+W_{air}=\frac{1}{2}m{v}^2$
$\therefore W_{air}=\frac{1}{2}m{v}^2-W_{mg}$
$=\frac{1}{2}m{v}^2-mgh$
$=\frac{1}{2}\times 5\times {\left(10\right)}^2-\left(5\right)\times \left(10\right)\times \left(20\right)$
$=-750J$