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.
Note.
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!
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.
