As said in the introduction, "dimensional homogeneity" is the quality of an equation having quantities of same units on both sides. A valid equation in physics must be homogeneous, since equality cannot apply between quantities of different nature. This can be used to spot errors in formula or calculations. For example, if one is calculating a speed, units must always combine to if one is calculating an energy, units must always combine to [mass•²/², etc. For example, the following formulae could be valid expressions for some energy: