The centre of gravity of an object is the point where all the weight of the object is considered to act.
- For a symmetrical rigid object, the centre of gravity lies at its centre.

The moment of a force is a measure of the turning effect of the force around an axis or pivot. It is calculated by multiplying the force by the perpendicular distance from the pivot to the line of action of the force.
- Mathematically,
  Moment of Force = Force × Perpendicular distance

Principle of Moment
- For any object that is in rotational equilibrium, the sum of the clockwise moments about any point is equal to the sum of the anticlockwise moments about that same point.
- In other words,if an object is in rotational equilibrium, then
  Clockwise moment = Anticlockwise moment

Application of Principle of Moments
- To check if an object will remain balanced or start to rotate.
  This can be done by calculating the total clockwise moment about a point and the total anticlockwise moment about the same point. If both the clockwise moment and the anticlockwise moment are equal, then the object is in a balanced position; otherwise, it will start to rotate.
- To calculate an unknown force or distance when the object is balanced.
When the either force or the distance from the pivot are not perpendicular to one another, we must account for the component of force which is perpendicular to the distance from the pivot when calculating the moment about the pivot.
In cases where two or more forces are acting at the same point, it is useful to find the net force first, before applying the principle of moments or calculating the moment of force.

3.05 - Turning Effect of Forces

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