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# Question and Answer

## QuestionPhysicsClass 12

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? $${\left({g}={9.8}{m}/{s}^{{{2}}}\right)}$$
(A) $${9.5}\%$$
(B) $${19}\%$$
(C) $${38}\%$$
(D) $${76}\%$$

## Answer

Answer: C
In first Case, $${W}_{{{1}}}=\frac{{1}}{{2}}{m}{\left({v}_{{{1}}}\right)}^{{{2}}}+{m}{g}{h}$$
$$=\frac{{1}}{{2}}{m}{\left({12}\right)}^{{{2}}}+{m}\times{10}\times{12}$$
=72m + 120 m
and in second case, $${W}_{{{2}}}$$ = mgh
=120 m
The percentage of energy saved
$$=\frac{{{192}{m}-{120}{m}}}{{{192}{m}}}\times{100}={38}\%$$
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