The rain drop of mass 1 g , starts with zero velocity from a height of 1 km . It hits the ground wit

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The rain drop of mass 1 g , starts with zero velocity from a height of 1 km . It hits the ground with a speed of $5	ext{ }m/s$. The work done by the unknown resistive force is $\_\_\_\_$ J . (take $g = 10	ext{ }m/s^{2}$ )

Accepted Answer

Sol. [Key Concept: Work-energy theorem: $mgh + W_{res} = \frac{1}{2}\ {mv}^{2}$, so $W_{res} = \frac{1}{2}\ {mv}^{2} - mgh$.]$${w_{g} + w_{	ext{res}	ext{~}} = \Delta k }{mgh + w_{	ext{res}	ext{~}} = \frac{1}{2}{mv}^{2} }{	herefore w_{res} = \frac{1}{2}{mv}^{2} - mgh }{= 10^{- 3}\left( \frac{25}{2} - 10^{4} ight) }{= - 9.98	ext{ }J }$$

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