Density: a key concept

What is density?

Density (denoted by ρ) measures the mass contained within a given volume of matter. It is calculated using the formula:

ρ = m / V

where m is the mass (in kg) and V is the volume (in m³). The unit is kg/m³ (g/cm³ is also used).

Some useful values

Substance	Density (kg/m³)
Air	approximately 1.2
Oil	approximately 920
Water (fresh)	1000
Wood (oak)	approximately 750
Ice	approximately 920
Iron / steel	approximately 7900

Compare to predict

To determine whether an object floats or sinks in a fluid, we compare its average density to that of the fluid:

• if rho_object < rho_fluid -> the object floats
• if rho_object > rho_fluid -> the object sinks
• if ρ_object = ρ_fluid -> the object remains in equilibrium, motionless between the two bodies of water

This is why ice (ρ approx. 920 kg/m³) floats on water (1000 kg/m³): it is less dense.

A common pitfall to avoid

Don’t confuse mass with density! A large boulder (with a high mass) can have the same density as a small pebble. What matters for floating is not the total mass, but the mass-to-volume ratio compared to that of the fluid. This is also why a huge steel container ship floats: its average density (hull plus the air inside) is much lower than that of solid steel.