Density compares mass with space — Physical education, 11–13 years
Density tells how much mass is packed into a certain volume. It helps explain why some objects float, sink or feel surprisingly heavy for their size.
How tightly matter is packed
Density compares an object’s mass with the space it takes up. A small lump of lead can have more mass than a much larger lump of wood because its matter is packed more tightly. Density is calculated by dividing mass by volume, usually in kilograms per cubic metre.
Why density helps
Looking only at size or mass cannot tell us what a material is like. Scientists needed a fair comparison between samples of different sizes, so they asked how much mass belongs to each unit of space. This helps identify materials and predict whether an object will float in a liquid.
Finding a block’s density
A metal block has a mass of 540 g and a volume of 200 cm³. Divide mass by volume: 540 ÷ 200 = 2.7 g/cm³. Water has a density of about 1 g/cm³, so this block is denser than water and would sink if it had no trapped air.
Big does not mean dense
A reasonable mistake is to call a large object dense because it feels heavy. But density is about how much mass fits into each part of its volume, not the object’s total mass. A huge empty cardboard box can have more volume and less density than a small solid stone.
Boats and materials
Steel is denser than water, yet a steel ship can float because its hollow shape includes lots of air. The ship’s average density, counting steel and air together, is less than water. Engineers use this idea when designing boats, life jackets and containers that must float.
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