Q: A 50 kg man is running at a speed of 18 km/h . If all the kinetic energy of the man can be used to increase the temperature of water from 20 °C to 30 °C, how much water can be heated with this energy?
(a) 5 g
(b) 10 g
(c) 15 g
(d) 20 g
Ans: (c)
Sol: v = 18 km/h = 5 m/s
Since all the kinetic energy of the man can be used to increase the temperature
$\large \frac{1}{2} m v^2 = m’ s \Delta T $
$\large \frac{1}{2} \times 50 \times ( 5)^2 = m’ \times 4200 \times (30-20) $
On solving ,
m’ ≈ 15 g