Centre of Gravity

The centre of gravity of an object is the point where all the weight of the object appears to act. It is in the same position as the centre of mass.

We can represent the weight of an object as a downward arrow acting from the centre of mass or gravity. This can also be called the line of action of the weight.

Balancing (Also seen in GCSE Physics 3)

When an object is balanced:

the total moments acting clockwise = the total moments acting anticlockwise

An object suspended from a point (e.g. a pin) will come to rest with the centre of mass directly below the point of suspension.

If the seesaw to the left is balanced then the clockwise moments must be equal to the anticlockwise moments.

Clockwise moment due to 3 and 4

Anticlockwise moments due to 1 and 2

So