Density — Physics, 14–17 years
Density tells us how much mass is packed into a certain volume. It helps explain why objects made from different materials can have the same size but different masses, and why some objects float.
What density means
Imagine packing grains into boxes. Density describes how tightly the matter is packed: the same-sized box can contain little mass if it holds air, or much more if it holds metal. We calculate it by dividing mass by volume.
Why measure density?
People needed to identify materials and check whether an object was really made of what it claimed to be. Mass alone is not enough: a large piece of wood may weigh more than a small piece of lead. Comparing mass with volume gives a fairer test.
A density calculation
A metal block has a mass of 540 g and a volume of 200 cm³. First write the rule: density = mass ÷ volume. Then calculate 540 ÷ 200 = 2.7, so its density is 2.7 g/cm³. The units matter because they describe the chosen measurements.
The bigger-object trap
A common mistake is to say that the heavier object must be denser. That feels reasonable because we often use “heavy” to mean “packed with matter”, but size also affects mass. Density compares mass with volume, so a large object can be heavier yet less dense than a small one.
Density in the world
Engineers use density when choosing materials for buildings, vehicles and aircraft: a strong, low-density material can reduce mass. Laboratories also compare measured density with known values to identify an unknown substance. In each case, density is useful because it links what an object weighs to how much space it occupies.
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