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A man throws the brick to a height of 12 m with a speed of 12 m/s. If he throws the brick in such a way that they just reach that height , what percentage of energy will be saved

\[(a)\;29\;\% \quad (b)\;38\;\% \quad (c)\;46\;\% \quad (d)\;50\;\%\]

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1 Answer

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 $=\large\frac{KE}{KE+PE}$$\times 100$
$\qquad= \large\frac{\Large\frac{1}{2}mv^2}{\Large\frac{1}{2}mv^2+mgh} $$\times 100$
$\qquad=\large\frac{12^2}{12^2+2 \times 9.8 \times 12}$$ \times 100$
$\qquad=38 \%$
Hence  b is the correct answer.


answered Jul 22, 2013 by meena.p
edited Feb 10, 2014 by meena.p

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