Density: why things float
Build blocks of cork, wood, ice, brick and iron, divide mass by volume, and see why less dense things float.
Standards: 5-PS1-3MS-PS1-2
1Wonder
A cube of cork and a cube of iron are exactly the same size, yet one is light as a feather and the other heavy as a stone. One floats, one sinks. What number tells them apart, and why does a giant log float while a tiny nail sinks?
2Explore
This activity needs a bigger screen
Turn your phone sideways, or open this lesson on a tablet or computer. The explanation and quiz below work here.
- Pick Wood and change the number of cubes. The mass changes, but does ρ change? Does the block still float?
- Try every material in Water. Which ones float? Compare each ρ with water’s.
- Find a material that floats in water but sinks in Oil.
- On the graph, where is the dot for every floater? For every sinker?
3Explain
Same size, different mass
A cube of cork and a cube of iron take up the same space, but the iron has about 30 times more mass. Its particles are heavier and packed more tightly.
Density = mass ÷ volume, measured in g/cm³. It depends on the material, not on how big the piece is. An object floats in a liquid that is denser than it, and sinks in one that is less dense.
4Practice
A block has a mass of 12 g and a volume of 4 cm³. What is its density?
5Recap
- Density = mass ÷ volume: how tightly mass is packed into space.
- Every piece of the same material has the same density, big or small.
- Less dense than the liquid → floats; denser → sinks.
- Liquids of different densities stack in layers, the least dense on top.