Q. A body of mass $1kg$ is thrown upwards with a velocity $20ms^{- 1}$ . It momentarily comes to rest after attaining a height of $18m$ . How much energy is lost due to air friction? $\left(g = 10 m s^{- 2}\right)$ :-

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A

$30J$

B

$40J$

C

$10J$

D

$20J$

Solution:

$K\cdot E_{i}=P\cdot E_{f}+W\cdot D_{f r .}$
$\frac{1}{2}\left(\right.1\left.\right)\left(\right.20\left(\left.\right)^{2}=\left(\right.1\left.\right)\left(\right.10\left.\right)\left(\right.18\left.\right)+W\cdot D_{f r}$
$200=180+W\cdot D_{f f}$
$K=\sqrt{1 . 4 }R$

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