## Physics-SchoolUK.com

### The UK site for KS3 and KS4 Physics

KS4 Work done and Energy transfer

You should already have worked through the Forces and their interactions section.

Now we are going to spend a short while looking at the second of those two great themes in Physics; forces was the first great theme; energy is the second.

So, let's continue with this section, Work done and energy transfer.

When a force is used to move an object, energy is transferred (because the object has moved) and we say that Work is Done. A Force that causes movement in the direction of the force has DONE WORK.

The bigger the force that is used, or the greater the distance moved, the greater the amount of Work is Done.

So, from this statement we can derive a simple equation, first in words:

Notice that the unit for Work Done or Energy is the "joule". Named after James Joule, an English Physicist.
For the unit symbol use a CAPITAL letter, J. Never use the lower case letter j for the unit symbol.

From the equation you should be able to see that one joule of work is done when a one newton force moves an object a distance of one metre.
So, 1 joule = 1 newton-metre
Or, 1 J = 1 Nm
So the newton-metre, Nm, is an alternative unit for Energy or Work Done.
BUT more importantly, the KS4 syllabus requires that for GCSE you be able to convert between joules and newton-metres.

Well, as we have just seen, it couldn't be easier,
1 J = 1 Nm,
So, convert 10.7 J into an equivalent number of newton-metres:
OK, easy.

Before we look at a few calculation examples, a few points to note regarding Work Done:

1. If a person applies a force to an object but the object does NOT move, then no work is done on the object, no matter how hard the person pushes or pulls. Zero movement, zero Work.
For example a weight lifter Does Work whilst applying the Force and lifting the weights, but once the weights have stopped moving, even though he or she is still "straining like mad" to hold up the weight, NO more Work is being Done! No Movement, No Work. There might be a Force (and a lot of sweating), but if there is NO movement in the direction of the force, then NO Work is Done!

Or, if a force is applied but the object does not move in the direction of the force, then once again, NO work is done! To Do Work, you would have to be raising and lowering your bag as you walked along. Just holding it level is NOT doing Work.

2. When Work is Done against frictional forces, such as pushing a box across a floor, energy is transferred from the person or thing doing the work, increasing the temperature of the moving object and the contact surface.

This is how most braking systems work - car, motorcycle and bicycle brakes all work by applying a braking Force which Does Work over a Braking Distance during which the vehicle slows down; at the end of the process the brake pads or discs will be very hot, so the Work Done by the braking force has caused an energy transfer, increasing the temperature of the pads and discs.

OK, let's have a go together at a few example calculations.