Moment of a force is a measure of its tendency to cause a body to turn about a specific amount. 

Moments = Force x Perpendicular distance
M = F x ∟d

Moments is measured in Newton metre (Nm)


Example: 

A force of 20 N acts at a perpendicular distance of 2 m from a pivot. Find the moments. 

Explaining moments

M = F x ∟d
M = 20 x 2 = 40 Nm


Well as you have already guessed, the questions are not going to be so simple. 

Moments question usually involve a beam being balanced on an axis or on a point. The beam must be in equilibrium, i.e. totally balanced and not tilting to one side. 

In order for the beam to remain completely balanced, one condition is needed to be satisfied:

Clockwise moments = Anti-clockwise moments

Let us understand this with and example. 


Example:

A beam is to remain in equilibrium. Here is the diagram: 
Find the mass of the beam. 

moments example

Solution:
Yes! This is a ridiculous question. But don’t worry!

Let the mass of beam = K

Anticlockwise moments are the moments of forces trying to move the beam in the anticlockwise direction.
The orange square is trying to move the beam in an anticlockwise direction. 
The orange square is applying a force of 100 N at a perpendicular distance of 4 m from the axis. 

Anticlockwise moments = F x ∟d
M (Anti-C) = 100 x 4 = 400 Nm

Clockwise moments are the moments of forces trying to move the beam in the clockwise direction. In this case it is the yellow circle. 
However! there is one more force acting in the clockwise direction and you may have guessed it. It is the weight of the beam. 
Weight of the beam = 10K N
Remember! The weight of a body always acts on its centre (centre of gravity).

Length of beam = 10 m
Centre of beam = 10 ÷ 2 = 5 m
Distance of centre from the axis = 5 – 4 = 1 m
Moment created by weight of beam = 10K x 1 = 10K Nm

Distance of yellow circle from axis = 10 – 4 = 6 m
Moment created by yellow circle = 50 x 6 = 300 Nm
Total M (Clockwise) = 10K + 300 

We must satisfy the condition: 

Clockwise moments = Anti-clockwise moments

400 Nm = 10K + 300 Nm
100 Nm = 10K
K = 100 ÷ 10 = 10 kg