Units are standards of measurement used for different types of quantities.

The most prevalent system of units is the SI (Système international d'unités). It consists of seven base units: meter (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), candela (luminous intensity), and mole (amount of substance). Units for other quantities are derived from these, for example velocity is $\text{m/s}$ or meter-per-second.

Units need not make any physical sense unless rearranged in the correct manner. For example, writing surface tension as $\mathrm{\frac{kg}{s^2\cdot m^2}}$ need not make any intuitive sense. If it is rewritten as $\mathrm{N \cdot m^{-1}}$, then we can clearly see that it has to do with force per unit length.

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