What is couple? Give its illustration.

Explanation

A pair of equal and opposite forces with different lines of action is known as a couple or torque. A couple produces rotation without translation.
Illustration 1 :

As according to above figure when we open the lid of the bottle by turning it, our finger are applying a couple to the lid. So it is in translational equilibrium but not in rotational equilibrium.
Illustration 2 : [Compass needle]

The earth.s magnetic field exerts equal forces on the north and south poles.
The Earth.s magnetic field exerts equal and opposite force on the poles of a compass needle.
The force on the North pole is towards the north and the force on the south pole is toward the south.
When a needle is in North-South direction, both forces have same line of action of force, but except this direction the line of action of force is not same. Hence, due to magnetic field of earth a couple acting on the needle. So it undergoes rotational motion without translation motion.