Work and power
If you and a friend climb the same stairs, who has more power?
Play
Explain
Pushing things is work.
Doing it fast takes more power.
In physics, work means a force moving something. No movement means no work, however tired you feel.
Work is measured in joules, because it is energy being moved.
Power is how fast you do work. Doing the same job in half the time takes twice the power.
W = F × d, where d is the distance moved along the direction of the force. Work equals energy transferred.
P = W / t. One watt (W) is one joule per second.
Worked example: 50 N pushes a box 10 m in 5 s. W = 500 J, and P = 500 / 5 = 100 W.
- Force moves something
- That is work
- Work per second is power
Want the full story?
Only the part of a force along the motion does work. Carrying a bag across a level floor at steady speed does almost no work on the bag, even though your muscles are using energy. A force at an angle θ does work F d cos θ.
The Formula
work = force × distance
- the work done, which is energy transferred, measured in joules (J)
- the force, measured in newtons (N)
- the distance moved along the force, measured in metres (m)
Try the numbers
Work done: 500 J
The Formula
power = work ÷ time
- the power, measured in watts (W)
- the work done, measured in joules (J)
- the time taken, measured in seconds (s)
Try the numbers
Power: 100 W
Try it at home
Ask a grown-up to help
You need a book, a stopwatch, some stairs.
You need: a book, a stopwatch, some stairs.
Lift a book up high. That is work!
- Weigh yourself and multiply your mass by 9.8.
- Measure the height of some stairs.
- Time yourself walking up carefully, and work out your power.
What's happening: work = weight × height, and power = work ÷ time. Do not run on stairs.
Physics is life
Your body
Your heart works hard all day.
Daily life
Light bulbs say how many watts.
Nature
Cheetahs have huge power.
Space
Rockets have huge power.
Quick check
Question 1 of 3
Words to know
Work: Energy transferred when a force moves something.