If you wanted to move a heavy rock across the bottom of a shallow pool, would it be easier to move it if the pool was empty or full of water?
The rock would be easier to move if there was water in the pool. The reason, buoyancy force.
Archimedes, a Greek mathematician was the first to develop a formal understanding of buoyancy by recognizing that an object placed in a liquid is buoyed up by a force equal to the mass* of the liquid displaced by the object. This is referred to as a buoyancy force. Buoyancy force is a function of the density* of an object relative to the density of the liquid in which it is immersed. If an object is less dense than the surrounding liquid, it will float. If it is more dense, it will sink.
The following illustration explores the relationship between an objects apparent weight* when submerged in a liquid as a function of its density and the density of the liquid it is placed in.
- Sea Water
- Mineral Oil
Density is a physical property of all matter. The more mass an object has in a given space (volume*) the more dense it is. The mathematical relationship that describes density is:
density = mass / volume
The standard units of density are g/cm3 or kg/m3. For a complete review of density and the calculation of buoyancy force see the first buoyancy interactive illustration.
Different materials have different densities and these differences are the bases of may commonly observed phenomena including oil floating on water and why the fat in chicken broth can be skimmed off the top.
Archimedes famously used the idea that an objects will displace its weight in water to compare the densities of two irregularly shaped objects. This principle, called Archimedes principle, can be used in a variety of ways. The density of a fluid can be determined by determining the apparent mass of an object if the density of the object is know. Or, the density of a solid object can be calculated by determining its apparent mass when submerged in a liquid of known density.