# Physical size

A physical size is a whole of measuring units, variables, of orders of magnitude and methods of measurement (which are the object of the Métrologie) related to an aspect or particular phenomenon of the Physique. For example, the size length gathers all that relates to the distances.

“The value of a size is generally expressed in the shape of the product of a number by a unit. The unit is only one particular example of the size concerned, used like reference. The number is the relationship between the value of the size in question and the unit. ”

The addition and the subtraction of numbers are not possible that if they are relative to the same size. On the other hand, it is possible to multiply or divide different sizes, in which case one obtains a new size derived from both others. For example, the Speed is resulting from the division of the Length by the Temps. There thus exists theoretically an infinity of sizes, but only a certain number of them are used in practice. The field of the physics which treats relations between the sizes is the dimensional Analyze.

## Fundamental sizes

The possibility of deriving from the sizes from others implies the existence of a starting point, in other words of fundamental sizes or basic sizes. These sizes, or rather their units, are often gathered in systems of units according to the utility of their relations and their combinations. The most widespread system currently is the international Système which rests on seven basic units.

The fundamental sizes are also closely related to particular fields of physics. An attempt at classification according to these fields is proposed below. The sizes mentioned are those of the international system. For each size is given its dimension within the meaning of the dimensional Analyze, as well as the corresponding unit of IF. The list is not exhaustive. The incorporation of the size angle in the dimensional analyzes is not general. The two approaches, with or without incorporation, are indicated.

### Mechanics

#### Fundamental

• Mass (M, kilogram)

#### Derived

• Pressure (ML-1T-2, Pascal)
• Density (ML-3, kilogram per cubic meter)
• energy (ml ² T-2, joule)
• Momentum (MLT-1, newton-second)
• Angular momentum (ml ² T-1α-1 or ml ² T-1)
• power (ml ² T-3, Watt)
• force (MLT-2, newton)
• Couple (mechanical) (ml ² T-2α-1 or ml ² T-2, newton-meter per radian or newton-meter)
• Action (physical) (ml ²T-1)

## References

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