In physics, a couple is a pair of equal and opposite forces that are parallel to each other but do not act along the same line.
Some of the criteria to form a couple are as follows:
- They should be equal in magnitude.
- They should act in the opposite direction.
- They should be separated by a certain distance.
  Here is an example of a couple acting in a circular disc.
The couple produces a pure turning effect. In other words, a couple will not accelerate the object, rather will rotate it.

The turning effect of the couple is measured in torque.
Torque is the product of one of the forces and the perpendicular distance between the forces.
Mathematically,
Torque (τ) = Force × Perpendicular distance (d)

An object is in translational equilibrium when the net resultant force exerted on it equals zero. When the net torque acting on an object is zero, the object is in rotational equilibrium. The object is in equilibrium when both the net resultant force and the net torque acting on the object are zero.
If three coplanar vectors acting on an object form a closed triangle, then the object is in equilibrium and the net force acting on it is zero.
When the forces acting on an object form a triangular loop, either clockwise or counterclockwise, with three coplanar vectors, the resulting force is zero, and the object is in equilibrium. It can be illustrated as shown:

3.06 - Equilibrium of Forces

Learn Step-By-Step Video Explanation For This Topic