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The Moment of a Force (also called torque)
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 = forcperpendicular 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 clock-wise moments about any point must have the same magnitude as the sum of all the anti-clockwise 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.  
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