Centre of Gravity

The centre of gravity of an object is defined as:

The point through which the weight of an object acts

For a symmetrical object of uniform density, the centre of gravity is located at the point of symmetry
- For example, the centre of gravity of a sphere is at the centre

Finding the centre of gravity of symmetrical objects

Point of symmetry

The centre of gravity of a regular shaped object can be found using symmetry

Examiner Tip

Since the centre of gravity is a hypothetical point, it can lie inside or outside of a body. The centre of gravity will constantly shift depending on the shape of a body. For example, a human body’s centre of gravity is lower when learning forward than when standing upright

However, when you are drawing force diagrams, always draw the weight force as if it were acting from the centre of gravity of the object!

Stability

The centre of gravity of a symmetrical object is along the axis of symmetry
The position of the centre of gravity affects the stability of an object
An object is stable when its centre of gravity lies above its base

Centre of gravity of an object in different positions

Stability, downloadable AS & A Level Physics revision notes

The object will topple, when its centre of gravity is no longer over its base

If the centre of gravity does not lie above its base, then an object will topple over
The most stable objects have a low centre of gravity and a wide base

Low centre of gravity of a car

Cars are stable because they have a low centre of gravity and a wide base

Taller objects with a narrow base have a higher centre of gravity and are less stable
This is why lorries and buses are advised not to use motorways and bridges on very windy days

Stability of object in different positions

Stability and base, downloadable AS & A Level Physics revision notes

When the object is positioned on its narrow base, it is less stable because its centre of gravity is higher

Moments and stability

If the line of action of the weight force lies outside the base of the object, there will be a resultant moment, and the body will topple

Car and bus on varying inclines

A car on various inclined planes up to 60 degrees without toppling because the line of action of its weight still lies within its base. A bus is tilted to 45 degrees before the line of action of its weight lies outside its base.

The car can be titled to 60° without toppling, but the bus will topple at 45°

Tall objects with a narrow base will topple easily
- Ten-pin bowling pins are designed specifically to topple easily
The stability of objects can be increased by widening the base
- High chairs are designed with a wide base so that they do not topple
- Bunsen burners have a wide base to ensure they do not topple

Centre of Gravity (CIE IGCSE Physics: Co-ordinated Sciences (Double Award))

Revision Note