 If the unit vector along the axis is formula_6, the moment of force about the axis is defined as
where formula_8 indicates the vector dot product.The adjacent figure shows a beam that is acted upon by a force formula_9.If the coordinate system is defined by the three unit vectors formula_10, we have the following 
Therefore,
The moment about the axis formula_13 is then

The negative value suggests that a moment that tends to rotate a body clockwise around an axis should have a negative sign.