Moments - Brief Introduction | Physics Grade XI

Moment of Force

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Moments - Brief Introduction | Physics Grade XI

Moments - Brief Introduction

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.

mathematical expression of moment of force

Clockwise and Anticlockwise Moments
As shown in the figure, from the point of rotation O, Forces F1 and F2 are at perpendicular distance of r1 and r2. The effect of force F1 is to rotate the body in an anticlockwise direction and that of F2 is to rotate in a clockwise direction. Therefore, the moment of force caused by F1 is an anticlockwise moment and F2 is a clockwise moment.                        
clockwise and anti-clockwise momentsPrinciple 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.

Like Forces

Unlike ForcesTorque 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.                    

Torque due to couple

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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