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