Principle of Moment

Principle of moment states that when a body is in equilibrium, the sum of clockwise moments about a pivot is equal to the sum of anticlockwise moments about the same pivot.

Conditions for equilibrium:
(1) Resultant force is 0
(2) Resultant moment about the pivot is 0 => principle of moment must apply.

Note:
(1) When solving questions on equilibrium, we have to always ensure these two conditions are satisfied.
(2) When solving questions on moment, always identify the position of pivot first and then label all the forces and their associated perpendicular distance. 
(3) When solving questions on moment, must always include the moment of the weight into the calculation unless weight is very small or it acts at the pivot. 

Example:

A uniform rod AB of weight 100 N and length 2 m is supported by two vertical strings at its ends and carries a load of 20 kg as shown in the diagram below. Determine the tensions in the strings.
Answer:
Let the tension of the string at A be T1
      the tension of the string at B be T2

 Taking moments at A
       100 x 1 + 200 x 0.5 =T2 x 2
                                   T2 = 100 N

Sum of upward forces = sum of downward forces
                           T1 + T2 = 200 + 100
                                   T1 = 300 – 100 = 200 N
OR
Taking moments at B
       100 x 1 + 200 x 1.5 =T1 x 2
                                   T1 = 200 N

Application: Levers

All levers have a pivot or fulcrum. When applying a force or effort at one end of lever, a load is lifted at the other end. e.g. wheelbarrow, hammers
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