Worked examples on power
The first runs the formula forwards and takes four lines to reach the work. The second runs it backwards, from power to work.
Example 1
The problem
The time taken to raise an object of mass 5 kg to a height of 8 m is 10 seconds. Calculate the rate of doing work, that is, the power.
mass of the object = 5 kg
weight of the object = m g = 5 kg × 10 m s-2 = 50 N
force needed to raise it = 50 N
height raised = 8 m
work done = 50 N × 8 m = 400 J
power = 400 J ÷ 10 s
= 40 W
Example 2
The problem
How much work does a machine with a power of 100 W do in one minute?
power = 100 W = 100 J s-1
time = 1 minute = 60 s
work done in 60 s = power × time = 100 J s-1 × 60 s
= 6000 J
Watch how the units cancel
Multiplying J s-1 by s leaves J, which is the reassurance that the second example was set up the right way round. If the units of your answer are not the units the question asked for, the arrangement is wrong.