A machine possesses 80 % efficiency. It raises a mass of 5 kg through a certain distance and spends 100 J of energy….

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

Click to See Answer :
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