Density — Physical education, 14–17 years
How mass is packed into a given volume, and why equal-sized objects can have very different masses.
The idea
Density tells us how much mass is packed into each unit of volume. A small block of lead can be heavier than a much larger block of wood because its particles are packed more closely. We calculate density by dividing mass by volume: ρ = m/V.
Why we need it
Mass alone does not tell us what a material is like: a lorry and a coin can both contain metal, but their masses differ because their sizes differ. Density gives a fair comparison between equal volumes. The idea became useful in measuring materials and checking whether objects were made from the substance claimed.
A worked example
A metal cube has a mass of 540 g and a volume of 60 cm³. First divide mass by volume: ρ = 540 ÷ 60 = 9 g/cm³. If an unknown sample also has density 9 g/cm³, it could be the same material, provided the measurements are reliable.
The common trap
A common mistake is to think that the biggest object must have the greatest density. That feels reasonable because a bigger object often has more mass, but density compares mass with the space occupied. A large sponge can have more mass than a small cork piece and still be less dense.
Outside school
Engineers use density when choosing materials for bridges, bicycles and aircraft: strength and low mass often matter together. In laboratories, density can help identify a liquid or check a metal sample. Whether an object floats also depends on comparing its average density with the density of the liquid around it.
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