Question

An object of mass $5kg$ falls from rest through a vertical distance of $20m$ and reaches a velocity of $10m/s$. The work done by the push of the air on the object $(g=10m/s^2)$

• A. $500J$
• B. $-750J$
• C. $-500J$
• D. $750J$

Answer 1

The correct option is D $750J$

In dropping by the given distance, the velocity that should have been attained by the object=$\sqrt{2gh}=\sqrt{2\times 10\times 20}=20m/s$

Thus the loss in kinetic energy of the object due to resistance by the air= Negative of work done by push of air$=\frac{1}{2}m{v}_{ideal}^2-\frac{1}{2}m{v}_{actual}^2$

$=\frac{1}{2}\times 5\times ({20}^2-{10}^2)$

$=750J$