The power formula
One formula, and most problems give you two of its three quantities and ask for the third.
Power
power = work done (J) ÷ time taken (s)
With the work in joules and the time in seconds, the power comes out in watts.
Which way to use it
Read the question first and find out which of the three is missing; that decides whether you divide or multiply.
| To find | When you are given | Do this |
|---|---|---|
| Power | Work and time | Divide the work by the time |
| Work done | Power and time | Multiply the power by the time |
| Time taken | Work and power | Divide the work by the power |
Time goes in as seconds
A question that says "in one minute" is giving you 60 s, and one that says "in two minutes" is giving you 120 s. Put the minutes straight into the formula and the answer is sixty times out.
Work often has to be found first
Most power questions do not hand you the work in joules. They give a mass and a height, or a force and a distance, and expect you to calculate the work before you can divide by the time at all.