# Centre of Mass

## Key Stage 4

File:BalancingBird.png
This toy balances by having its centre of mass directly over its beak.

### Meaning

The centre of mass is the average position all the matter in an object.

The centre of mass is also sometimes called the centre of gravity which is the point in an object where weight seems to act from.
When calculating forces and drawing free body diagrams objects can be modelled as if they were a sphere at the centre of mass.
The centre of mass of a lamina object can be found by hanging it from a point. The centre of mass will be directly below the point from which it is hung. Hanging it from a second point the centre of mass will also be below this new point. Those lines will cross over at the centre of mass.

### Method to find the Centre of Mass of a Lamina

1. Hang the lamina from a point.
2. Hang a plumb line from that point.
3. Draw a line down the lamina following the string of the plumb line.
4. Hang the lamina from a second point.
5. Repeat steps 2 and 3.

The point where the lines cross will be the centre of mass.

A third line can be drawn to confirm this location.

### References

#### AQA

Centre of mass, pages 157, 169, GCSE Physics; Student Book, Collins, AQA
Centre of mass, page 120, GCSE Physics, Hodder, AQA
Centre of mass, page 129, GCSE Combined Science Trilogy; Physics, CGP, AQA
Centre of mass, page 149, GCSE Physics; The Complete 9-1 Course for AQA, CGP, AQA
Centre of mass, page 204, GCSE Combined Science; The Revision Guide, CGP, AQA
Centre of mass, page 52, GCSE Physics; The Revision Guide, CGP, AQA
Centre of mass, pages 124-125, GCSE Physics; Third Edition, Oxford University Press, AQA
Centre of mass, pages 210, GCSE Combined Science Trilogy 2, Hodder, AQA