Centres of Gravity
Every object has a balance point. You can support it on a finger, it being not too heavy, and if, with you finger positioned in a certain place, the object will balance. Your finger will exert an upwards force on the object to stop it falling. Opposing this upwards force is the weight of the object, which acts downwards. Though the body may be made up of lots of individual atoms, the weight of it can be considered to act through a single point. This point is called the centre of gravity.
Given any body, the position of the centre of gravity can be found by hanging it from several different points. When it is hanging from any point, the position of the centre of gravity will be below the balance point. Draw a line from this point vertically down. Do this for several hanging points (P,Q and R in the diagram below) and the intersection of these lines will be the centre of gravity (C.G.).
It is important to realise the centre of gravity does not have to be within the object. The centre of gravity of a boomerang is outside the boomerang, as shown below.