Density
Density is mass per volume: ρ = m/V.
About this topic
Density is the mass per unit volume, ρ = m/V, in kg/m³ or g/cm³. Water is 1000 kg/m³, aluminium about 2700 and air about 1.2. Density is a property of the material, not the amount: a tiny gold ring and a bar have the same density.
To find it, measure mass with a balance and volume by dimensions for regular shapes or by displacement of water for irregular ones. Density explains why some things float and others sink, why oil sits on water, and why hot air rises. Gold at about 19,300 kg/m³ is nearly 19 times denser than water, and the Earth overall averages 5500 kg/m³.
Worked example: a metal block of mass 540 g occupies 200 cm³, so its density is 540 / 200 = 2.7 g/cm³, matching aluminium. A 2 m³ tank of petrol with density 740 kg/m³ has a mass of 1480 kg. Be careful with units: 1 g/cm³ equals 1000 kg/m³.
Engineers use density to choose light, strong materials for aircraft, and geologists identify minerals by it. Jewellers detect fake gold with it, as in Archimedes' famous crown story. A misconception is that density is the same as weight; a large block of polystyrene is much heavier than a small lead ball but far less dense.
Key ideas
- Water is 1000 kg/m³
- Denser things sink in lighter fluids
- Units kg/m³
Quick check
Use cases: where Density helps
- Homework: review the key idea behind Density before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Density without notes.
- Visual learning: play with the interactive lab to see how Density changes when you adjust its inputs.
- Teaching a friend: explain Density 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 Density?
Density is mass per volume: ρ = m/V.
What will I learn in Density?
Water is 1000 kg/m³. Denser things sink in lighter fluids. Units kg/m³.
Is Density 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 Density 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 Density?
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.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.