Work

There are a number of words which have a very specific meaning in science but also have a different colloquial meaning. This can be a source of confusion for those studying physics.

If I was to say I had a weight of 80 kg not only would I be lying I would also be technically incorrect. I should say I have a weight of 800N as weight is a force. mass, not weight, is measured in kilograms.

Another such word is work.

Work is done when the point of application of a force moves. The amount of work (in J) is equal to the product of the force and the distance moved in the direction of the force. basically...

Work = Force x Distance

consider these examples.

A man pushes a car with a force of 500N for a distance of 30m. How much work does he do?

W = F x d = 500 x 30 = 15,000J

notes
i) The work done will be equal to the work against friction plus the gain in k.e. of the car.
ii) If the car doesn't move then the person is NOT actually doing any useful work, no matter how exhausted they might feel afterwards.

A crane lifts a beam of mass 200kg a height of 20m at a steady speed. How much work does it do?

W = F x d = (200 x 9.8) x 20 = 39,200J

notes
i) Notice that the answer is equal to the g.p.e. gained by the beam.
ii) If the lift was not done at a steady speed then the change in k.e. would need to be considered.
iii) When an object falls gravity does work pulling it down.

A force of 50N is needed to keep a bicycle moving at a steady speed of 10m/s. How much work does the rider have to do every second?

W = F x d = 50 x 10 = 500J

notes
i) The amount of work done by the rider of every second is his useful power output
ii) It is because of friction that the rider must work to keep going at the same speed. Friction is doing work trying to slow him down.

Work can almost be regarded as a type of energy. In energy equations "work done by ..." of ten appears. If fact work is energy on the move. Whenever work is done an energy transfer is taking place. Consider the examples above and you will see this to be the case.