Buoyancy and Archimedes' Principle
The buoyant force equals the weight of fluid displaced.
About this topic
Archimedes' principle says that the upward buoyant force on a submerged object equals the weight of fluid it displaces: F = ρ_fluid · V_displaced · g. The force arises because pressure is greater on the bottom of the object than on top.
An object floats when its average density is less than the fluid's, and a floating object displaces its own weight of fluid. A steel ship floats because its hollow shape displaces a huge volume of water, giving a low average density. Hot air balloons and airships float in air by the same principle, and submarines control depth by taking in or pumping out water ballast.
Worked example: a 0.002 m³ block fully under water feels a buoyant force of 1000 × 0.002 × 9.8 = 19.6 N. If the block weighs 15 N it will float up until it displaces 15 N of water, and if it weighs 30 N it sinks, appearing to weigh 10.4 N underwater. The Dead Sea is so salty and dense that people float easily.
Archimedes reputedly used the idea to test whether a crown was pure gold. A common misconception is that heavy objects sink and light ones float; what counts is density, not weight. Another is that the force depends on depth, but it does not, as long as the object is fully submerged.
Key ideas
- Floats if density is less than the fluid
- Ships float by displacing a lot of water
- Apparent weight loss = buoyant force
Quick check
Use cases: where Buoyancy and Archimedes' Principle helps
- Homework: review the key idea behind Buoyancy and Archimedes' Principle before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Buoyancy and Archimedes' Principle without notes.
- Visual learning: play with the interactive lab to see how Buoyancy and Archimedes' Principle changes when you adjust its inputs.
- Teaching a friend: explain Buoyancy and Archimedes' Principle in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Buoyancy and Archimedes' Principle?
The buoyant force equals the weight of fluid displaced.
What will I learn in Buoyancy and Archimedes' Principle?
Floats if density is less than the fluid. Ships float by displacing a lot of water. Apparent weight loss = buoyant force.
Is Buoyancy and Archimedes' Principle part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Buoyancy and Archimedes' Principle lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Buoyancy and Archimedes' Principle?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
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