Question

(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/s²)

Answer 1

(a) Power is defined as rate of doing work. It is also defined as rate at which energy is consumed. SI unit of power is watt.

(b) Mass of the boy, (m₁) = 40 kg
Mass of the box, (m₂) = 20 kg
Height, (h) = 15 m
Acceleration due to gravity, (g) = 10 m/s²
Time taken, (t) = 25 s
Total mass of the system,
M = m₁ + m₂
=> M = 40 + 20 = 60 kg
So, work done by the boy is,
W = mgh
=> W = (60)(10)(15)
=> W = 9000 J
Now, we can calculate the power of the boy as,
$$\begin{gathered} \mathrm{Power}=\frac{\mathrm{Work}\mathrm{done}}{\mathrm{Time}} \\ \mathrm{So}, \\ \mathrm{Power}=\frac{9000}{25}\mathrm{W} \\ =\boxed{360\mathrm{W}} \end{gathered}$$
So, the power is 360 W.