Consider again the couple comprising two equal and opposite parallel forces of magnitude P and distant d apart.

 

 

 

 

 

 

 

 

 

 

 

 

We will take moments about three different axes, through A, B and C

clockwise moment of A = P( a + d ) -Pa = Pd

clockwise moment of B = P( d - b ) + Fb = Pd

clockwise moment of C = P( c + d ) - Pc = Pd

As A, B and C represent any points in the plane of the couple, we see that

then moment of a couple is the same

about any axis perpendicular to its plane

The magnitude of the moment, or torque, of a couple is often called simply the magnitude of the couple.