# Upthrust

## Key Stage 3

### Meaning

Upthrust is a force that acts upwards in a fluid due to the fluid being displaced.

Upthrust is a force so it is measured in Newtons.
The force of upthrust is equal to the weight of fluid that has been displaced by an object.
An object in the water experiences more upthrust than an object in air because water is more dense than air.
Upthrust is weak in the air because the air is not very dense. For most objects in air upthrust can be ignored because it is so small.
Upthrust is caused by the pressure in a fluid. Pressure increases with depth so the bottom of an object will experience more pressure than the top. This difference in pressure results in a difference between the force on the bottom and the top, known as upthrust.

### Examples

 A boat floats on the water because the weight is balanced by the upthrust. A submarine can change the amount of upthrust in order to rise or sink in the water.

## Key Stage 4

### Meaning

Upthrust is a force that acts upwards in a fluid due to the weight of the fluid being displaced.

Upthrust is a force so it is measured in Newtons.
The force of upthrust is equal to the weight of fluid that has been displaced by an object.
An object in the water experiences more upthrust than an object in air because water is more dense than air.
Upthrust is weak in the air because the air is not very dense. For most objects in air upthrust can be ignored because it is so small.
Upthrust is caused by the pressure in a fluid. Pressure increases with depth so the bottom of an object will experience more pressure than the top. This difference in pressure results in a difference between the force on the bottom and the top, known as upthrust.
Upthrust = [(Pressure at base) - (Pressure at the top)] x (Area of top and bottom surface)

### Examples

 A boat floats on the water because the weight is balanced by the upthrust. A submarine can change the amount of upthrust in order to rise or sink in the water.

### References

#### AQA

Upthrust, page 137, GCSE Physics, Hodder, AQA
Upthrust, page 173, GCSE Physics; Student Book, Collins, AQA
Upthrust, page 59, GCSE Physics; The Revision Guide, CGP, AQA
Upthrust, pages 168-169, GCSE Physics; Third Edition, Oxford University Press, AQA
Upthrust, pages 172, 173, GCSE Physics; The Complete 9-1 Course for AQA, CGP, AQA

#### Edexcel

Upthrust, page 102, GCSE Physics; The Revision Guide, CGP, Edexcel
Upthrust, pages 204-205, GCSE Physics, Pearson Edexcel
Upthrust, pages 321, 322, GCSE Physics, CGP, Edexcel

#### OCR

Upthrust, page 19, Gateway GCSE Physics; The Revision Guide, CGP, OCR
Upthrust, pages 42, 63, Gateway GCSE Physics, Oxford, OCR