Moments

Moments are also known as ‘turning force’ or ‘torque’. These forces take place about a pivot.

Moment = Force x Perpendicular distance of the line of action of the force from the pivot

           

 

                                                                                                                                                      

 

Moment (Nm)
Force (N)
Perpendicular distance (m)

 

The principle of moments

1. For any object that is at equilibrium the sum of the clockwise moments is equal to the sum of the anticlockwise moments about the same point.
2. For a body to be at equilibrium the resultant force on it is zero.

 

Example applying the principle of moments

The following object is at equilibrium. Find the angle between 6N to the vertical.

 

 

 

 

 

 

Taking moments about the pivot

Sum of the anticlockwise moments = Sum of the clockwise moments

0.75 x 10 = 8 Cos(x)
Cos-1(7.5 / 8) = 20.36 ͦ

Centre of Gravity
The centre of gravity is defined as the point at which the entire weight of a body can be considered to act.

Couple
A couple is a pair of forces equal in magnitude and opposite in direction, which produce rotation but not acceleration.

 

 

 

 

The moment of a couple = Force x Perpendicular distance