Question

A ball of mass $5\text{kg}$ is dropped from a tower. Find the power of gravitational force at time $t=2\text{seconds}$.
(Take $g=10{\text{m/s}}^2$)

• A. $1000\text{W}$
• B. $2000\text{W}$
• C. $100\text{W}$
• D. $200\text{W}$

Answer 1

The correct option is A $1000\text{W}$

Given, mass of ball $m=5kg$
Gravitational force on the ball;

$F=mg=5\times 10=50\text{N}$

$\therefore$ Instantaneous power $P$ =$Fv\cos \theta$

To find $v$: since $a$ is constant
we use, $v=u+at\Rightarrow v=gt=10\times 2=20m/s$

$\therefore P=\left(50\right)\left(20\right)\cos 0^{\circ }$
$($as $F$ and $v$ are in same direction$)$
$P=1000\text{W}$

Hence option A is the correct answer