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.
Read more about this topic: Physical Quantity
Famous quotes containing the words definition of a, definition of, definition, physical and/or quantity:
“Definition of a classic: a book everyone is assumed to have read and often thinks they have.”
—Alan Bennett (b. 1934)
“One definition of man is “an intelligence served by organs.””
—Ralph Waldo Emerson (1803–1882)
“The definition of good prose is proper words in their proper places; of good verse, the most proper words in their proper places. The propriety is in either case relative. The words in prose ought to express the intended meaning, and no more; if they attract attention to themselves, it is, in general, a fault.”
—Samuel Taylor Coleridge (1772–1834)
“Be sure that it is not you that is mortal, but only your body. For that man whom your outward form reveals is not yourself; the spirit is the true self, not that physical figure which can be pointed out by your finger.”
—Marcus Tullius Cicero (106–43 B.C.)
“All things will be produced in superior quantity and quality, and with greater ease, when each man works at a single occupation, in accordance with his natural gifts, and at the right moment, without meddling with anything else.”
—Plato (c. 427–347 B.C.)