Physics Topic
Density
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Study focus - 3 things
- Density tells you how much mass is packed into a certain volume
- The formula is density = mass / volume
- Density calculations need correct units, usually g/cm³ or kg/m³
Section 1What density means
Density tells you how tightly matter is packed. A small metal bolt can feel heavier than a large sponge because the metal has more mass packed into each cubic centimetre. In plain words, density compares mass with volume.
Section 2The formula
The density formula is density = mass / volume. You can rearrange it if needed: mass = density x volume, and volume = mass / density. This is one of the most common exam slip-ups, so write the formula before substituting numbers.
Section 3Units
If mass is in grams and volume is in cubic centimetres, density is in g/cm³. If mass is in kilograms and volume is in cubic metres, density is in kg/m³. Do not mix units without converting.
Section 4Density in labs
For regular solids, measure length, width, and height to find volume. For irregular solids, use displacement. Then measure mass with a balance and calculate density.
Examples
An object has mass 60 g and volume 20 cm³. Find its density.
Density = mass / volume = 60 / 20 = 3 g/cm³.
A small piece of metal from a lab kit has mass 78 g. It displaces 10 mL of water. What is its density?
The volume is 10 cm³ because 10 mL = 10 cm³. Density = 78 / 10 = 7.8 g/cm³.
Now let's water it
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