How to play Density Detective
- Choose an object and read its mass, its density, and the cylinder's readings on the card.
- Use ρ = m ÷ V. Under water an object pushes aside its own volume, so V = V₁ − V₀, and 1 mL = 1 cm³.
- Type your prediction and press Lower. The object goes in steadily and the water rises.
- Watch the orange line: it shows your prediction next to what the water really does.
What is density?
Density tells you how much mass is packed into each cubic centimetre: ρ = m ÷ V. Water's is about 1 g/cm³, so 1 mL of water has a mass of about 1 g; aluminium's is 2.7 g/cm³ and gold's 19.3 g/cm³. Mass is easy to measure on a scale, but the volume of an odd shape, such as a crown or a stone, is hard to work out. Archimedes' trick: sink it in water and read how far the level rises. An object under water pushes aside exactly its own volume, and 1 mL = 1 cm³. Divide the mass by that volume and you have the density, which identifies the material: a 'gold' crown with a density of 16 g/cm³ has been mixed with a lighter metal. Density also decides what floats. An object sinks through a liquid less dense than itself and floats on one denser than itself, which is why oil sits on water and honey sinks below it.
A focused learning model
Each object hangs on a thread and is lowered steadily for 2 s, from just above the water to where it comes to rest. The cylinder's cross-section A is the same all the way up, so pushing aside ΔV mL raises the level by ΔV ÷ A cm. Objects are solid and soak up no water; the block, the crown, the stone, and the bar sink to the bottom and end fully under water, so each pushes aside its own volume. In the column the layers don't mix, and the bead is small enough that its own volume is ignored; it rests on top of the first layer denser than itself. Heights are to scale. A prediction is right within 2%. Plays in real time.