# PlanetPhysics/Mass

Mass is often denoted by the letters ${\displaystyle m}$ or ${\displaystyle M}$. The SI unit for mass is the kilogram (kg). One kilogram is defined as the mass of the international prototype of the kilogram , which is made from an alloy of platinum and iridium and is kept in the Bureau international des poids et mesures in Paris.
In classical mechanics, there are two kinds of mass. The first one, called inertial mass , appears in the best-known version of Newton's second law, ${\displaystyle F=ma}$. This law says that the force ${\displaystyle F}$ needed to give an object an acceleration ${\displaystyle a}$ is proportional to ${\displaystyle a}$, the proportionality constant being defined as the inertial mass of the object. The other kind of mass, gravitational mass , is the mass occurring in Newton's law of gravitation, ${\displaystyle F=GM_{1}M_{2}/r^{2}}$. (We use the symbol ${\displaystyle M}$ to indicate the difference with the inertial mass, ${\displaystyle m}$.) This equation states that to any object we can associate a quantity ${\displaystyle M}$, the gravitational mass, such that the gravitational force between two objects with gravitational masses ${\displaystyle M_{1}}$ and ${\displaystyle M_{2}}$ at a distance ${\displaystyle r}$ is proportional to ${\displaystyle M_{1}M_{2}/r^{2}}$.