39. A man throws the bricks to the height of 12 m, where they
reach with a speed of 12 m/s. If he throws the bricks such that
they just reach this height, what percentage of energy will he
save?
(a) 19% (b) 76% (c) 38% (d) 57%
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Explanation:
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When the brick reaches 12 m height when thrown with velocity 12 m/s it has both potential energy and kinetic energy
When it just reaches the same height, it has only Potential energy, as its velocity is zero at its maximum height.
Therefore %% of energy saved =KEKE+PE=KEKE+PE×100×100
=12mv212mv2+mgh=12mv212mv2+mgh×100×100
=122122+2×9.8×12=122122+2×9.8×12×100×100
=38%
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