Question

A ball of mass 1 kg is dropped from a height of 5m find
(i) K.E. of the ball as it is $\frac{1}{2}$ way to the ground.
(ii) Find P.E. at this instant.

Answer 1

Solution

(i) $W_{gravity}=-mg△h=mg(h_i-h_f)$

$=\left(1\right)\left(10\right)[5-1/2]=45J$

By work - Energy Theorem,

$W_{net}=△KE$

$\Rightarrow W_{gravity}=\frac{m}{2}[v_f^2-v_i^2]$

$\Rightarrow 45=\left(\frac{m}{2}\right)[v^2-(0{)}^2]$

$\Rightarrow \frac{m{v}^2}{2}=45J$

Find KE of the ball is 45J.

$\left(ii\right)PE=-mgh$

$=-\left(1\right)\left(10\right)\left(1/2\right)=-5J$