moment arm

The resultant of non-concurrent force system is defined according to magnitude, inclination, and position.
 

The magnitude of the resultant can be found as follows

Couple is a system of forces whose magnitude of the resultant is zero and yet has a moment sum. Geometrically, couple is composed of two equal forces that are parallel to each other and acting in opposite direction. The magnitude of the couple is given by
 

 
Where $F$ are the two forces and $d$ is the moment arm, or the perpendicular distance between the forces.
 

Moment is the measure of the capacity or ability of the force to produce twisting or turning effect about an axis. This axis is perpendicular to the plane containing the line of action of the force. The magnitude of moment is equal to the product of the force and the perpendicular distance from the axis to the line of action of the force. The intersection of the plane and the axis is commonly called the moment center, and the perpendicular distance from the moment center to the line of action of the force is called moment arm.