# 4.2.5 Archimedes Principle

### Archimedes' Principle

• Archimedes’ principle states that an object submerged in a fluid at rest has an upward buoyancy force (upthrust) equal to the weight of the fluid displaced by the object
• The object sinks until the weight of the fluid displaced is equal to its own weight
• Therefore the object floats when the magnitude of the upthrust equals the weight of the object
• The magnitude of upthrust can be calculated by:

Upthrust equation

• Since m = ρV, upthrust is equal to F = mg which is the weight of the fluid displaced by the object
• Archimedes’ Principle explains how ships float:

Boats float because they displace an amount of water that is equal to their weight

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
Close