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P5 F) Forces Doing Work
P5 F) Forces Doing Work
When a force causes an object to move a distance, work is done and energy is transferred. We can work out energy transferred by multiplying the force by the distance.
In the equation above, force is measured in Newtons (N) and distance is measured in metres (m) [the distance is the distance moved along the line of action of the force]. Work done is measured in Joules and 1 Joule is 1 Newton metre (1 J = 1 Nm).
The Energy Transfer
Work is done when we move an object because energy is transferred from one energy store to other energy stores that are both useful and wasteful. In order to cause an object to move or continue moving a force must be applied. Whatever is applying the force needs a source/ store of energy, which will be food for living organisms (humans) or a fuel for machines (cars). When a force moves an object, work is done against the frictional/ resistive forces opposing the movement.
Let’s have an example of me pushing a toy car along a carpet. When I am pushing a toy car across the carpet, I am doing work against the frictional/ resistive forces that are acting in the opposite direction to the force that I am applying to the toy car. When I am pushing the toy car, energy is being transferred from me (the individual pushing the toy car) to the kinetic energy stores of the toy car (the useful energy transfer) and to the thermal energy stores of both the carpet/ surroundings and the toy car (the wasted energy transfer). After I have finished pushing the toy car along the carpet, the temperature of both the carpet and the toy car that have been in contact with each other will increase (the thermal energy stores of the toy car and the carpet will increase).
Work is done when we move an object because energy is transferred from one energy store to other energy stores that are both useful and wasteful. In order to cause an object to move or continue moving a force must be applied. Whatever is applying the force needs a source/ store of energy, which will be food for living organisms (humans) or a fuel for machines (cars). When a force moves an object, work is done against the frictional/ resistive forces opposing the movement.
Let’s have an example of me pushing a toy car along a carpet. When I am pushing a toy car across the carpet, I am doing work against the frictional/ resistive forces that are acting in the opposite direction to the force that I am applying to the toy car. When I am pushing the toy car, energy is being transferred from me (the individual pushing the toy car) to the kinetic energy stores of the toy car (the useful energy transfer) and to the thermal energy stores of both the carpet/ surroundings and the toy car (the wasted energy transfer). After I have finished pushing the toy car along the carpet, the temperature of both the carpet and the toy car that have been in contact with each other will increase (the thermal energy stores of the toy car and the carpet will increase).
Example 1
A 40 N force pushes a toy car 3 metres. Work out the work done.
We work out the work done by multiplying the force by the distance.
A 40 N force pushes a toy car 3 metres. Work out the work done.
We work out the work done by multiplying the force by the distance.
We are told in the question that the force is 40 Newtons and the distance is 3 metres. Both of these values are in the correct units, so we just sub them into the equation.
The work done is 120 Joules (which can also be written as 120 Nm).
Example 2
9 Joules of work is done to push an object 45 centimetres. Find the force that is applied to the object.
We find the calculation that we undertake to find the force by covering up f in the formula triangle. When we do this, we see that we can find force by dividing the work done by the distance.
9 Joules of work is done to push an object 45 centimetres. Find the force that is applied to the object.
We find the calculation that we undertake to find the force by covering up f in the formula triangle. When we do this, we see that we can find force by dividing the work done by the distance.
Work done in this calculation needs to be in joules or Newton metres, which it is; the work done is 9 joules. The distance needs to be in metres, which it is not; the distance is 45 centimetres. We can convert centimetres to metres by dividing by 100; the distance in metres is 0.45 m (45 ÷ 100 = 0.45). We now have all of the values in the correct units; we sub in the work done as 9 Joules and the distance as 0.45 metres.
The force applied to the object is 20 Newtons.