Centre of Gravity (Cambridge (CIE) IGCSE Physics)
Revision Note
Written by: Leander Oates
Reviewed by: Caroline Carroll
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
The centre of gravity of a regular shaped object can be found using symmetry
Examiner Tips and Tricks
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
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
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
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
Examiner Tips and Tricks
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 upright
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