The moment of a force is the turning effect that it has. It is measured in newton metres (Nm)
The ability of a force to make an object turn depends on TWO factors:
- the size of the force that acts at right-angles to a line through the turning point of the object you wish to turn
- the distance the force is applied from the turning point.
A small force can have the same effect as a big one if it is applied a greater distance from the fulcrum or turning point. So you can use the moment principle to maximise the effect of a force you use to move something.
When the force is supplied by a weight its centre of gravity is the point from which the distance the force acts is measured.
Be careful not to call the moment a 'turning force' - the turning force is only part of the moment... the moment is the effect that the turning force has on the system! If it were just a force it would be measured in N... the moment is measured in Nm which has the same dimensions as energy.
At GCSE they expect you to define the distance in a particular way - so learn the phrasiology by heart!
Moment = force × perpendicular distance from the line of action of the force to the axis of rotation
That's quite a mouthful - but learn it!
The Principle of Moments
For a body in equilibrium (blalnced!) the sum of the clockwise moments is equal to the sum of the anticlockwise moments.
clockwise moments = anticlockwise moments
How to tackle questions:
- draw a diagram and mark on all of the information you have been given and allocate the unknown a symbol - in an examination annotate the one on the paper.
- Identify the 'pivot point', 'turning point' or fulcrum - the point around which the whole system turns.
- Identify all of the forces acting. If they act through the support you can ignore them (as the support will produce a reaction force that will cancel them out!) - you may have to calculate some of the forces as they may not be given to you - you may be given 'mass' instead of weight, for example.
- calculate the perpendicular distance from each of the forces to the turning point - in advanced level questions you are rarely given the correct distance!
- work out all of the clockwise moments and add them together
- work out all of the anticlockwise moments and add them together
- state the Principle of Moments
- equate the clockwise and anticlockwise moments
- find the unknown
- check that this unknown is actually the value you are asked for in the question - sometimes they ask you for a distance that requires this information before you can do the last step!
- check that you have included the corect unit in your answer and that it is to the correct number of significant figures.
