Question:
(a) $W=F \times S=m g S=250 \times 10 \times 1=25.0 \mathrm{~J}$.
(b) Zero. This is because displacement of box is zero.
(c) They get tired because muscular force applied by them is needed to balance the weight of the box.
Solution:
The Jog Falls in Karnataka State are nearly $200 \mathrm{~m}$ high. 2000 tonnes of water falls from it in a minute. Calculate the equivalent power if all this energy can be utilized ? $\left(g=10 \mathrm{~ms}^{-2}\right)$
Here, $b=200 m, m=2000 \times 1000=2 \times 10^{6} \mathrm{~kg}$
$g=10 \mathrm{~ms}^{-2}, t=1$ minute $=60 \mathrm{~s}$.
Power $=\frac{\text { work }}{\text { time }}=\frac{m g h}{t}=\frac{2 \times 10^{6} \times 10 \times 200}{60}$
$=6.67 \times 10^{7}$ watt.