Question

A $100\text{kg}$ block is dropped from the top of a building $80\text{m}$ tall. Assume $g=10{\text{ms}}^{-2}$. Find the work done by the force of gravity as it falls vertically to hit the ground.

• A. $80000\text{J}$
• B. $-80000\text{J}$
• C. $20000\text{J}$
• D. $0\text{J}$

Answer 1

The correct option is A $80000\text{J}$

We know that,
Workdone, $W=\overrightarrow{F}.\overrightarrow{s}$

$F=mg=100\times 10=1000\text{N};s=80\text{m}$
$W=FS\cos \theta$, as it is falling vertically the force of gravity is along the direction of displacement $(\cos 0^{\text{o}}=1)$

$W=FS\cos 0^{\circ }=FS=1000\times 80=80000\text{J}$

Hence option A is the correct answer