Question
Define couple. Derive an expression for the moment of couple.
Solution
Couple is the set of two equal and opposite force having a different line of action. A couple produces the torque and tends to cause the rotational motion.
Consider two equal and opposite forces acting at points A and B having position vectors 
as shown in figure. 
Moment of force 
about O is, 
where 
is between F and r and d is the perpendicular distance between the lines of action of two forces of couples.
Thus, the magnitude of the moment of couple is equal to magnitude of one of the force of couple and perpendicular distance between the lines of action of two forces of couple.
