An unruly demonstrator lifts a stone of mass 200 g from the ground and throws it at his opponent. At the time of projection, the stone is 150 cm above the ground and has a speed of 3 m/s. Calculate the work done by the demonstrator during the process. If it takes one second for the demonstrator to lift the stone and throw it, what horsepower does he use?

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Solution

$Given, Mass of the stone, m = 200 g = 0.2 kg Height to which the stone is lifted, h = 150 cm = 1.5 m Velocity of the projection, ν = 3 m / s Time, t = 1 s Total work done, W = K . E . + P . E . W = \frac{1}{2} m ν^{2} + m g h = (\frac{1}{2}) \times (0.2) \times 9 + (0.2) (9.8) \times (1.5) = 3.84 J$

1 hp = 764 watt

Horsepower used by demonstrator

$= \frac{3.84}{746} = (5.14) \times 10^{- 3}$

Therefore, power used by the demonstrator to lift and throw the stone is 5.14 $\times$ 10^-3 hp.

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