The moment of a force (or the torque produced by a force) is a measure of
the turning effect of the force. 



Consider a (not very bright) person trying to open a door, by applying a
force, of magnitude, F, as shown below. 



Practical experience tells us that this person would find it easier to open
the door (held closed by a spring) if he/she 

1. pushed at 90° to the door and 

2. applied the force further from the hinge 

as shown below in the next diagram 





We conclude that the turning effect or torque of a force depends on the
magnitude of the force and the perpendicular distance between the line of
action of the force and the pivot (the hinge of the door in the case above). 

We therefore define the moment (or torque) to be 

moment = force×perpendicular distance of force from pivot 



and we see that the units of moment (or torque) are Newtonmeter,
Nm. 



However, note that this Nm is not the same as
the Nm for work (or energy). 

In other words, this Nm is not the same as the Joule (why not?
answer at bottom of page...) 



Returning to look at the first situation again in a little more detail,
we can obtain a more general equation for when the angle is not 90° 



It is clear that the perpendicular distance between the pivot and the force
is 



so, in this case, the moment is given by 





The concept of moment of a force is useful when considering situations in
which a body is acted on by a number of forces which are in equilibrium.


That is, a number of forces whose effects all cancel out. 

See here for more
on equilibrium. 



Considering the person pushing on the door above: 

If the person pushes on the door (imagine it to be half open for this
illustration), but the door does not move, then the turning effect (moment)
due to the spring must be just equal in magnitude but opposite in sense of
the turning effect due to the person pushing the door. 



This leads to the principle of moments, which can be stated as
follows: 

If a body is in (rotational) equilibrium, the sum of all the clockwise moments about any point must
have the same magnitude as the sum of all the anticlockwise moments
about the same point. 



N.B. 

In more advanced work you will discover the idea of the vector nature of
torque. 

In fact, the equation defining moment (torque) above should be
changed into a vector product… in which case the order of writing
the vectors multiplied becomes significant. 

So, the more complete definition of torque, usually represented by the Greek
letter τ is 





See, for example, here for more detail. 





This Nm is not equivalent to a Joule because here the distance and the force
are at 90° to each other. 
