**Coplanar Forces **are those forces that act in a single plane or same plane.

The product of force and moment arm is called **moment of force or torque**.

**Clockwise and Anticlockwise Moments**

As shown in the figure, from the point of rotation O, Forces F

_{1}and F_{2}are at perpendicular distance of r_{1}and r_{2}. The effect of force F_{1}is to rotate the body in an anticlockwise direction and that of F_{2}is to rotate in a clockwise direction. Therefore, the moment of force caused by F_{1}is an anticlockwise moment and F_{2}is a clockwise moment.**Principle of moment**

It states that when a body in equilibrium under the action of a number of forces, then the algebraic sum of the moments of all the forces at any point is zero i.e. sum of clockwise direction is equal to anticlockwise directions.

**Parallel forces**

When a number of forces act on a body and their directions are parallel, they are called parallel forces. They are of two types (i) like parallel forces and (ii) unlike parallel forces.

**Torque due to a couple**

Two equal and opposite forces acting on a body such that their lines of action do not coincide constitute a couple. Example: turning a tap, turning a corkscrew, etc.

In the figure, both forces cause anticlockwise rotation on the rod Ab about O. So, the moment of a couple is the sum of the moment of the two forces.

Torque about O = Moment of force F + Moment of force F at B

= F * AO + F * OB

= F * (AO + OB)

Τ = F * AB

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