(a) W = F x S = mgS= 250 x 10 x 1 =25.0 J.


(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.


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.

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