Definition of A Physical Quantity
Formally, the International Vocabulary of Metrology, 3rd edition (VIM3) defines quantity as:
Property of a phenomenon, body, or substance, where the property has a magnitude that can be expressed as a number and a reference.Hence the value of a physical quantity q is expressed as the product of a numerical value Nq and a unit of measurement uq;
Quantity calculus describes how to perform mathematical manipulations of quantities.
Examples
- If the temperature T of a body is quantified as 300 kelvin (in which T is the quantity symbol, 300 the value, and K is the unit), this is written
- T = 300 × K = 300 K,
- If a person weighs 120 pounds, then "120" is the numerical value and "pound" is the unit. This physical quantity mass would be written as "120 lb", or
- m = 120 lb
- If a person traveling with a yardstick, measures the length of such yardstick, the physical quantity of length would be written as
- L = 36 inches
- An example employing SI units and scientific notation for the number, might be a measurement of power written as
- P = 42.3 × 103 W,
In practice, note that different observers may get different values of a quantity depending on the frame of reference; in turn the coordinate system and metric. Physical properties such as length, mass or time, by themselves, are not physically invariant. However, the laws of physics which include these properties are invariant.
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