#### Power

Power is the rate at which energy is used or the rate at which work is done.

Mathematically, the above statement can be expressed as:

In symbols, this expression is written as:

The unit of power is the WATT, (W)

1 W = 1 J/s

**Example:**

A crane lifts a 250 Kg crate of roof shingles from the ground on top of a 15 m high roof in 20 s. How much power did the crane use up?

**Given: **

m = 350 Kg

g = 9.8 m/s^{2}

h = 15 m

t = 20 s

**Find:**

a) Force required

b) Work Done

c) Power

**Solution:**

a) The force Required to Lift Shingles up is the force of gravity, **F _{g }**

F

_{g}= m x g (the force of gravity)

= 350 Kg x 9.8 N/Kg

= 3430 N

b) Work Done = Work Done in lifting

= Fg x h

= 3430 N x 15 m

= 51 450 Nm

= 51 450 J

c) Power = Work / time

= W / t

= 51 450 J / 20 s

= 2573 W