A mass distribution can be modeled as a measure. This allows point masses, line masses, surface masses, as well as masses given by a volume density function. Alternatively the latter can be generalized to a distribution. For example, a point mass is represented by a delta function defined in 3-dimensional space. A surface mass on a surface given by the equation f (x, y, z) = 0 may be represented by a density distribution g (x, y, z) δ (f (x, y, z)), where g / | ∇ ∇ f | is the mass per unit area.