Cyberphysics - Moments and Levers

Moments and Levers

Click here for Simple Machines

How to 'teeter-totter' or 'see-saw' with someone bigger than you!

Moments in physics have nothing to do with time. 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 perpendicular 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.

You can use the moment principle to maximise the effect of a force you apply. These videos illustrate this:

When the force is supplied by a weight its centre of gravity is the point from which the force arrow is drawn. It goes vertically down. To find the perpendicular distance for use in the calculation of the moment, you need to draw a perpendicular line from the weight arrow to the line that passed through the fulcrum (axis of rotation!). (You sometimes have to draw dashed construction lines to do this!).

Be careful not to call the moment just 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! - in calculations you can get away with a shortened version:

Moment = Fd

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

Garfield has considerably more weight than Odie - so for them to 'teeter-totter' Odie has to sit a bigger distance away from the fulcrum. To play 'see-saw' their moments have to be equal (according to the Princlple of Moments):

F₁d₁ = F₂d₂

As Odie has less weight (force) he needs more distance!

How to tackle questions:

Draw a diagram and mark on all of the information you have been given and allocate the unknown a symbol (usually F or d) - 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.

Here is a practical exercise carried out by students to make a human mobile...

Here is a video to make you think about moments:

Click here to look at stability

Here is a link to a sample grade 9 KS3 SATs question on moments.

This is an interactive PowerPoint for you to work through

Try this interactive puzzle or this one!

Here is a lecture and demonstration - worth watching