A ball is thrown vertically downwards from a height of 20 m with an initial velocity v 0 . It collides with…

1. $20\mathrm{ }\mathrm{m }{\mathrm{s}}^{-1}$
2. $28\mathrm{ }\mathrm{m }{\mathrm{s}}^{-1}$
3. $10\mathrm{ }\mathrm{m }{\mathrm{s}}^{-1}$
4. $14\mathrm{ }\mathrm{m }{\mathrm{s}}^{-1}$

Solution

kinetic energy of the ball on reaching ground 
$K{E}_1=mgh+\frac{1}{2}m{v}_0^2$
after collision $\frac{K{E}_1}{2}=mgh$
$\frac{mgh+\frac{1}{2}m{v}_0^2}{2}=mgh$
$v_0=\sqrt{2gh}=20\text{m}{\text{s}}^{-1}$

Asked in: NEET 2015 (Phase 2)

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