9.1) Centre of gravityEQUILIBRIUM POINTAll objects have an equilibrium point.Equilibrium point is the point at which the

whole weight of the object may be considered to act.

In a regular polygon the equilibrium point lies at the intersection of its diagonals.

How about irregular polygons??

CENTRE OF GRAVITYThe equilibrium point of an object is also its

centre of gravity.The centre of gravity of an object is the point at

which the Earth’s gravity acts on that object.A regular object has its point of equlibrium and

centre of gravity at its centre.Centre of gravity of some objects does not lie

inside the objects.

B) Stability and factors affecting stability

Stability refers to an object’s ability to remain its original position.

An object is stable if it does not fall over/ topple easily.

An object is said to be in STABLE EQUILBRIUM if it returns to its original position when slightly displaced and then released.

It is in UNSTABLE EQUILBRIUM if it continues to move further from its original position after being displaced and then released.

It is in neutral equilibrium if it remains in its position even after being displaced.

Stability of an object is affected by : (i) its centre of gravity – the lower the centre

of gravity , the more stable the object is. (ii) its supporting base – the larger the base,

the more stable the object is Therefore a stable object has a low centre of

gravity and a large supporting base.

9.2) Importance of stability A) Ways to improve stability Two ways to improve stability, (i)Lowering its centre of gravity~This can be done either by reducing the

height or attaching a heavy weight onto its base

(ii) increasing its supporting base.

Applying the principle of stability Racing car is made with wide and spaced wide apart

to give a large supporting base. It is also designed with very low centre of gravity.

A giraffe will spread out its legs to increase the base area and to lower its centre of gravity and make itself stable when it bends its head to drink water.

The passengers of a double-decker bus are not allowed to stand on the upper deck. This is to lower the centre of gravity.

Heavy vehicles such as lorries and buses, their engines are located at the bottom of the vehicles. This is to lower the centre of gravity.