A ball of mass $50 g$ is thrown upwards. It rises to a maximum height of $100 m$ . At what height its kinetic energy will be reduced to $70$ %?

30 m

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40 m

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60 m

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70 m

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Solution

The correct option is A $30 m$
$H = \frac{u^{2}}{2 g} = 100$

Kinetic energy is zero and potential energy is maximum $= m g H$ at highest point.

Let at the height $h$ , kinetic energy of the ball is reduced to $70$ %

$h = \frac{u^{2}}{2 g} - \frac{v^{2}}{2 g} = 0.3 \frac{u^{2}}{2 g} = 30 m$

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