ES-80 Moment About A Line
    
    
The moment about a line is defined as shown
where
     - unit vector along the line
     - moment about any point on the line
Note: the moment about the line is a scalar.

The geometric configuration is shown here...

    
where
    
The moment about a line may be given in component form as shown here.

A = (2,3,1)
B = (3,-1,4)
C = (-2,1,-1)
For example, if a force is applied at point A and the moment is desired about line BC where A, B and C have coordinates as shown and where distances are in meters and forces are in newtons.







The moment may be found about any point on a line. Let us consider B. Then


The unit vector points from B to C because it was calculated from



The moment may be calculated...

This represents a moment of 5.99 about line BC. The negative sign indicates that the moment is in the opposite direction of , therefore pointing from C to B.

The moment about a line is important in that it allows us to calculate the torque about a shaft. Since we wish to transmit along the shaft, we would like all of the moment to be about that line. Any moments not about that line are deleterious.

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