Power is the rate at which energy is transferred or work is done. It has units of watts (W) or joules per second (J/s). The more energy that is transferred per second, or the more work that is done per second, the greater the power rating.

Calculating power

To calculate power, use the equation:

power = \frac{work~done}{time~taken}

This is when:

  • power is measured in watts (W)
  • work done is measured in joules (J)
  • time taken is measured in seconds (s)


A horizontal force of 50,000 N acts on a car, causing it to move a horizontal distance of 80 m in a time of 25 s. What is the power of the car?

First, find the work done on the car:

work done = force × distance

work done = 50,000 × 80

work done = 4,000,000 J

Then, find the power:

power = 4,000,000 ÷ 25

power = 160,000 W


A motor applies a force of 800 N to a trolley, causing it to move 80 m in a time of 16 s. What is the power of the motor?

Work done = 800 × 80

Work done = 64,000 J

Power = 64,000 ÷ 16

Power = 4,000 W

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