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What is density?

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Quick answer

Density is how much mass is packed into a given space — mass divided by volume. It's why a small steel ball sinks but a huge ship floats, and why oil floats on water: less dense things rise, denser things sink.

Key things to understand

  1. How much mass fits in a given volume.
  2. Density = mass ÷ volume.
  3. Denser-than-water things sink; lighter ones float.
  4. Explains floating, sinking, and why oil sits on water.

People also ask

How do you calculate density?
Divide an object's mass by its volume (density = mass ÷ volume); common units are g/cm³ or kg/m³.
Why do some things float?
An object floats when it's less dense than the liquid around it, so the liquid can hold its weight up.
Does temperature change density?
Yes — most things expand when heated, spreading the same mass over more volume, so they become less dense.

Part of the guide: Everyday Science: Light, Heat & Matter →