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Moments, Torque and Levers

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).

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, but the moment is measured in Nm which has the same dimensions as energy.

 

Torque is another term for ' moment', or moment of force in physics - it is the tendency of a force to cause or change the rotational motion of a body. It is a twist or turning force on an object. But in mechanics the word 'torque' is used in the case of a couple (only studied at A Level) as a special case of moment.

Moment is calculated by multiplying the magnitude of the applied force and the perpendicular distance of its line of action from the turning point.

It is a vector quantity - meaning it has both a direction and a magnitude.

The concept originated with the studies of the usage of levers by Archimedes.

Just as a linear force is a push or a pull, a torque is a twist to an object around a specific axis.

The symbol for torque is typically τ - the lowercase Greek letter tau.

The symbol for the moment of force, it is commonly denoted by M.

 

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.

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

Click here for Simple Machines - including the lever.

Click here for the Principle of Moments

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

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:



 

Click here to go to the page on the Principle of Moments

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