Question

An object of mass $5kg$ falls from rest through a vertical distance of $20m$ and acquires a velocity of $10m{s}^{-2}$ the work done by the push of air on the object is ($g=10m{s}^{-2}$)

• A. $750J$
• B. $-750J$
• C. $250J$
• D. $-250J$

Answer 1

The correct option is D $-250J$

Given,

$m=5kg$

$h=20m$

$v=10m/s$

$g=10m/s^2$

From work energy theorem,

$\Delta K.E=$ work done by air+ work done by weight

The work done by air, $W_a=\frac{1}{2}m{v}^2-mgh$

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

$W_a=-750J$

The correct option is B.