A **physical quantity** is a physical property of a phenomenon, body, or substance, that can be quantified by measurement. A physical quantity can be expressed as the combination of a magnitude expressed by a number – usually a real number – and a unit; for example, 9275×10^{−27} kg (the 1.674mass of the neutron), or 792458 metres per second (the 299speed of light). Physical quantities are measured as where is the magnitude and is the unit. For example: A boy measured the length of a room as 3 m. Here 3 is the magnitude and m (metre) is the unit. 3 m can also be written as 300 cm. The same physical quantity can be represented equivalently in many unit systems, *i.e.* .

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