Work
In physics work is done when a force moves an object in the direction of the force. Pushing a heavy box across the floor is work. Holding it still is not, because nothing moves.
- work done (J)
- force (N)
- distance moved (m)
Worked exampleLifting a bucket
A girl lifts a 50 N bucket up 3 m. How much work does she do?
- Force needed = weight = 50 N. Distance = 3 m.
- Calculate.
Answer: She does 150 J of work.
Energy
Energy is the ability to do work. It is also measured in joules. Two common forms of mechanical energy are kinetic energy (energy of motion) and potential energy (stored energy, for example because of height).
Try itA falling ball
A 2 kg ball is dropped from 10 m. Slide its height. Potential energy turns into kinetic energy and the total stays constant (air resistance ignored).
Power
Worked examplePower of a machine
A motor lifts a load and does 6 000 J of work in 20 s. What is its power?
- Use P = W ÷ t.
Answer: The power is 300 W.
Key terms
- Work
- Force times the distance moved in its direction.
- Energy
- The ability to do work, in joules.
- Power
- Work done per second, in watts.
- Conservation of energy
- Energy is never lost, only changed in form.
Summary
- Work W = F × d (joules).
- Kinetic energy ½mv²; potential energy mgh.
- Energy is conserved: it changes form but the total is constant.
- Power P = W ÷ t (watts).