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Centre of gravity

The attraction of the Earth's gravity acts on all bodies and we will now look at where this attraction acts.

Think of any object and consider it to be made up of millions of tiny particles, each one being acted upon by gravitational attraction. Now imagine all these tiny forces added together to make one force called the weight of the body.

The point in the body that the force acts on is known as the CENTRE OF GRAVITY of the body.

A body behaves as if all its mass was concentrated at one point called its centre of gravity

If the object is symmetrical like a ball or a broom handle then the centre of gravity is in the middle of the object.

Diagram (i) shows a uniform rod, the centre of gravity is at C, while for the non-uniform rod in diagram (ii) it is at H as one half of the rod is more massive than the other.

A good example of a non-uniform shape is a broom or a spoon. The mass on either side of their centre of gravity is not equal the distribution of this mass must also be allowed for.

You can also speak of the centre of mass of a body - this coincides with its centre of gravity in a uniform gravity field like that near the surface of the Earth. This will not be true near a Black hole where the gravitational pull changes rapidly with distance!

An object will only balance in equilibrium if the point of balance is directly below the centre of gravity. There is no resulting turning moment.
If it is not the case the object will fall as shown in the diagrams below.

This zero moment is also important when an object is suspended. It will only hang in equilibrium if the centre of gravity is vertically below the point of suspension. If it isn't the object will swing back.

The centre of gravity of a flat object can easily be found by hanging it up from three different points and marking the verticals each time.
Where these lines cross is the centre of gravity.

We can change the position of our centre of gravity to keep us balanced by stretching out our arms or by bending our bodies one way or the other.

In the pictures some of the people will fall over. Their arms are stretching out forwards and to prevent themselves from toppling they need to counterbalance this by sticking their bottom out backwards.

© Keith Gibbs 2020