Consider a couple as shown in Fig. 7.22 acting on a rigid body. The forces F and -F act respectively at points B and A. These points have position vectors `r_(1) " and " r_(2)` with respect to origin O. Let us take the moments of the forces about the origin.
The moments of the two forces making the couple
`=r_(1)xx(-F)+r_(2)xxF`
`=r_(2)XXF-r_(1)xxF`
`(r_(2)-r_(1))xxF`
But `r_(1)+AB=r_(2)` and hence `AB=r_(2)-r_(1)`. The moment of the couple, therefore, is `ABxxF`.
Clearly this is independent of the origin, the point about which we took the moments of the forces.
