Question

A body of mass $2\text{kg}$ is thrown vertically upwards with an initial speed of $10\text{m/s}$. The work done by the force of gravity during the time the body reaches its maximum height. (Consider $g=10{\text{m/s}}^2$)

• A. $100\text{J}$
• B. $0\text{J}$
• C. $-100\text{J}$
• D. $20\text{J}$

Answer 1

The correct option is C $-100\text{J}$

Given, mass of the body $\left(m\right)=2\text{kg}$
Initial speed $\left(u\right)=10\text{m/s}$

We know that maximum height reached by the body $\left(H\right)=\frac{u^2}{2g}=\frac{(10{)}^2}{2\times 10}=5\text{m}$
Force of gravity acts vertically downwards and the body is moving up
$\therefore$ Force of gravity is anti parallel to displacement
Then,
Work done by gravity $\left(W\right)=\overrightarrow{F}.\overrightarrow{s}=-Fs$. As, $\cos {180}^{\text{o}}=-1$.
The only force acting is weight $\left(mg\right)$, so
$W=-mgH$,
$W=-2\times 10\times 5=-100\text{J}$

Hence option C is the correct answer