Q: A machine possesses 80 % efficiency. It raises a mass of 5 kg through a certain distance and spends 100 J of energy. If the mass is now released, its kinetic energy on reaching the ground will be
(a) 100 J
(b) 80 J
(c) 20 J
(d) 10 J
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Ans: (b)
Sol: Energy Spent $\displaystyle = 100 \times \frac{80}{100} = 80 J$
If mass is released , applying conservation of energy ,
$\displaystyle mgh = \frac{1}{2}mv^2 $
$\displaystyle 80 = \frac{1}{2}mv^2 $
K.E = 80 J