Power: power = work done / time

The idea

This statement expresses power as the rate of doing work. Work done is the energy transferred when a force moves an object, measured in joules (J), and time is measured in seconds (s). Power is therefore in watts (W), where 1 W = 1 J/s. For example, lifting a 20 N box through 3 m does 60 J of work; if this takes 4 s, the power is 60 J ÷ 4 s = 15 W. The equation is the same relationship as P = E / t because work done and energy transferred are equivalent. To solve problems, calculate work done using W = F × s when needed, then divide by time. Rearranged, work done = power × time and time = work done / power.

What earns the marks

• Define work done as energy transferred when a force moves an object, measured in joules (J).
• State that power is the rate of doing work and is measured in watts (W), where 1 W = 1 J/s.
• Use power = work done / time, substituting work done in joules and time in seconds.
• Calculate work done first using W = F × s when the question gives force and distance rather than energy.
• Rearrange to work done = power × time or time = work done / power when required.

Common mistakes and how to put them right

• Forgetting to calculate work done from force and distance before dividing by time.

  Put it right: First find W = F × s.

• Mixing up work done with power.

  Put it right: Work done is in joules, power is in watts.

• Using distance instead of time in the denominator.

  Put it right: The denominator must be time in seconds.

Exam tips

• Underline the quantity the question asks for so you choose the correct rearrangement.
• Show the work-done calculation separately if force and distance are given, then divide by time.
• Include the unit W with your final answer and check it is reasonable for the device described.