#### Power

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

Mathematically, the above statement can be expressed as

In symbols, this expression is written as

and the unit of power is the WATT (W)

1 Watt is equal to one Joule per second

**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?

Solution:

Given: m = 350 Kg

Find: a) Force required (See also Forces)

g = 9.8 m/s2

b) Work Done (See also Energy and Work)

h = 15 m

c) Power

t = 20 s

Solution: a) Force Required to Lift Shingles up = Fg

= 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