Question

On a certain planet, a stone weighing $2\text{kg}$ is thrown vertically downwards with a velocity of $5\text{m/s}$ from a height of $30\text{m}$. It took $4\text{s}$ to reach the ground. Calculate the work done by gravity.

• A. $-75\text{J}$
• B. $60\text{J}$
• C. $-60\text{J}$
• D. $75\text{J}$

Answer 1

The correct option is D $75\text{J}$

It is given that: $S=30\text{m},u=5\text{m/s}$
$t=4\text{sec}$

$\therefore$ By kinematic $e{q}^n$, as it is dropped the acceleration due to gravity $\left(g\right)$

$S=ut+\frac{1}{2}g{t}^2$, we get the value of $g$ as,

$30=5\times 4+\frac{1}{2}\times g\times 16$
$\Rightarrow g=\frac{10}{8}=1.25{\text{m/s}}^2$

$\therefore$ work done by gravity $W_g=mg.s$
$=2\times 1.25\times 30$
$W_g=75\text{J}$

(Here the work done is positive as the force and the displacement are in the same direction)

hence option D is the correct answer