It is tempting to think that some materials are always heavier than other materials. For example, someone might say that metal is heavier than air. But it depends on how much of each material there is ...

Density describes how closely packed the particles are in a solid, liquid or gas. There is little difference between the density of a liquid and its corresponding solid (eg water and ice). This is ...

A planet’s mass and volume are easy to determine. Because planets are nearly spheres, we can calculate volume from our knowledge of a planet’s radius, which we determine by measuring both the planet’s ...