Moment vs Couple
Moment of force and couple are two important concepts found in mechanics. These describe the effect and cause of rotation in force systems, particle systems, and in rigid bodies. Moment of a force governs many of the rotational properties of the systems. From a certain perspective, it is the equivalent of the force in rotational dynamics.
Moment or more precisely the moment of a force is a measure of the turning effect of a force. Moment of force is measured in Newton meters (Nm) in the SI system, which looks similar to the unit of mechanical work but carries completely different meaning.
When a force is applied it creates a turning effect about a point other than on the line of action of the force. The amount of this effect or the moment is directly proportional to the magnitude of the force and the perpendicular distance to the force from the point.
Moment of a force = Force × Perpendicular distance from the point to force
Moment τ = F × x
If a force system has no resultant moments, i.e. ∑τ = 0 , the system is in rotational equilibrium.
When the moment of a force has a physical sense it is often called “torque”.
When two equal and opposite forces, but with separate lines of action are present in a force system it is called a couple. Both forces create their own moment of force, but the net moment of the couple is independent of the location of the point considered.
The moment of a couple is given by;
Moment of a couple = magnitude of force × perpendiclar distance between the forces
Even though the expressions are similar for both the moment of a force and of a couple, the physics behind are different.
Only one force is taken into account, though there are two forces in the couple. The turning effect of one force is countered by the other. Therefore, only the difference in the distance from the considered point accounts for the net turning effect. Hence the moment of the couple is a constant for any point on the plain of the couple.
Whenever a force is applied to create a turning effect, in reality a torque is produced by a couple. For example, consider using a wrench to unscrew a bolt. When the force is applied to the end of the wrench arm, a force with the same magnitude is created at the bolt, which is the pivot in this case. These two equal and opposite forces create a couple, and the couple is generating the torque required to turn the bolt.
What is the difference between Moment and Couple?
• Moment of force is the measure of turning effect of a force about a point. A couple consists of two equal and opposite forces acting with two different but parallel lines of action. Each force has its own moment.
• Moment of a force is dependent on the distance from the pivot and the magnitude of the force while the moment of a couple is the net effect of the two moments of the forces. Moment of a couple is independent of the location of the point considered. It is constant throughout the plane.