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# Couple and rotational equilibrium

A couple is a pair of equal forces acting on different points on an object so they cause a resulting moment but no net force. That means a couple changes the rotation of an object but it does not change its translational velocity.

When you use a screwdriver, you apply forces with your hand so that the overall location of the screwdriver does not change but it turns on the spot.

When there are forces acting on an object but no net torque, the object is said to be in rotational equilibrium. This means that the clockwise and anticlockwise moments cancel each other out exactly.

$F_1$ on the left acts alone and will cause the object to rotate if pivoted at A. The couple of $F_2$ and $F_3$ causes a rotation around B but does not cause any translation.