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  4. A man throws the bricks to a height of 12 m where they reach with a speed of 12 m / s. If he throws the bricks such that they just reach that height, what percentage of energy will be saved: ( Take g =9.8 m / s 2 )

Q. A man throws the bricks to a height of $12 \,m$ where they reach with a speed of $12 \,m / s$. If he throws the bricks such that they just reach that height, what percentage of energy will be saved : ( Take $g =9.8 \,m / s ^{2}$ )

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Solution:

Case-I.
image
Let speed with which the ball is thrown up is $u_{1}$
$(12)^{2} =u_{1}^{2}-2 \times 9.8 \times 12$
$u_{1}^{2} =379.2$
$K E_{1} =\frac{1}{2} m u_{1}^{2}=\frac{1}{2} m(379.2)$
Case-II.
image
$(0)^{2} =u_{2}^{2}-2 \times 9.8 \times 12 $
$u_{2}^{2} =235.2$
$K E_{2} =\frac{1}{2} m u_{2}^{2}=\frac{1}{2} m (235.2) $
$\%$ energy charge $=\frac{K E_{2}-K E_{1}}{K E_{1}} \times 100 $
$=-38 \%$
Negative sign shows that energy is saved.

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