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:

Power.5

In symbols, this expression is written as:

Power.6

The unit of power is the WATT, (W)    

note  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/s2    

h = 15 m   

t =  20 s

Find:

a) Force required 

b) Work Done 

c) Power

        

Solution: 

a) noteThe force Required to Lift Shingles up is the force of gravity, F
                     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