Doing work

when you push or pull something Doing work: when you push or pull something and make it move, you do work.

work done (in joules) = force (in newtons)  distance (in metres) How much work? Imagine pushing a car along a road. You must apply a force . . . . . . and you push the car a certain distance. work done (in joules) = force (in newtons)  distance (in metres)

the amount of work you do = the amount of energy transferred As you push the car, you transfer energy from your body to: the car – increasing its kinetic energy the surroundings (and bits of the car) – which heat up because of friction the amount of work you do = the amount of energy transferred

An example I push my car 30 m. I push with a force of 400 N. work done = force  distance = 400 N  30 m = 12 000 J

Doing work against gravity If you lift something up, you must exert enough force to balance the force of gravity. The upward force is equal to the weight of the object. You transfer some energy from your body – and this increases the gravitational energy of the thing you lift.

Work done = energy transferred So if you lift something through a certain distance work done = force  distance or change in gravitational potential energy = weight  vertical height gain

Lifting A woman lifts up her baby. The baby weighs 100 N. The woman lifts the baby up by 2 m. change in gravitational potential energy = work done = weight  vertical height difference = 100 N  2 m = 200 J

If you push your car uphill, you do work: work done = force  distance Pushing a car uphill If you push your car uphill, you do work: work done = force  distance You increase the gravitational potential energy of your car: change in gravitational potential energy = weight  vertical height gain Are these the same? If not, why not?

The calculation work done in pushing = force  distance = 450 N  30 m = 13 500 J gain in gravitational potential energy = weight  vertical height gain = 900 N  1 m = 9000 J

So why the difference? Some of the energy is transferred to the surroundings – heating them up. Only part of it goes to increasing the gravitational potential energy of the car.