(a) Define power. Give the SI unit of power.
(b) A boy weighing 40 kg carries a box weighing 20 kg to the top of a building 15 m high in 25 seconds.
Calculate the power. $(g=10\ m/s^2)$

(a). Power: The rate of the work done per unit time is called the power.  It is denoted by $P$. SI unit of power is $Joule/second$ or $Watt$. 

$\boxed{Power=\frac{work\ done}{time}}$

(b). Here mass of the boy $m_1=40\ kg$

Mass of the box $m_2=20\ kg$

Total mass $m=m_1+m_2=40\ kg+20\ kg=60\ kg$

Gravitational acceleration $g=10\ ms^{-2}$

Height $h=15\ m$

Time $t=25\ s$

Work done to lift the box $=mgh$

$=60\ kg\times10\ ms^{-2}\times15\ m$

$=9000\ J$

Therefore, power $=\frac{work\ done}{time}$

$=\frac{9000\ J}{25\ s}$

$=360\ W$

 So, $360\ W$ power is used to lift the box.

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Updated on: 10-Oct-2022

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