ISO 31-0-1992 pdf download – Quantities and units — Part 0: General principles
REMARK ON NOTATION FOR NUMERICAL VALUES It is essential to distinguish between the quantity it- self and the numerical value of the quantity expressed in a particular unit. The numerical value of a quantity expressed in a particular unit could be indicated by placing braces (curly brackets) around the quantity symbol and using the unit as a subscript. It is, how- ever, preferable to indicate the numerical value ex- plicitly as the ratio of the quantity to the unit. EXAMPLE A/nm = 589,6 NOTE 1 in the headings of columns in tables. This notation is particularly useful in graphs and 2.2 2.2. Quantities and equations Mathematical operations with quantities Two or more physical quantities cannot be added or subtracted unless they belong to the same category of mutually comparable quantities. Physical quantities are multiplied or divided by one another according to the rules of algebra; the product or the quotient of two quantities, A and B, satisfies the relations AB = í A W 1 – [Al PI Thus, the product {A){B) is the numerical value (AB} of the quantity AB, and the product [A] [BI is the unit [AB] of the quantity AB. Similarly, the quotient {A)/[B) is the numerical value { A p ] of the quantity A/B, and the quotient [A]/[B] is the unit [A/B] of the quantity A/B. EXAMPLE The speed v of a particle in uniform motion is given by v = r/t where i is the distance travelled in the time- interval t.
2.2.2 Equations between quantities and equations between numerical values Two types of equation are used in science and tech- nology: equations between quantities, in which a let- ter symbol denotes the physical quantity (¡.e. numerical value x unit), and equations between numerical values. Equations between numerical val- ues depend on the choice of units, whereas equations between quantities have the advantage of being in- dependent of this choice. Therefore the use of equations between quantities should normally be preferred. EXAMPLE A simple equation between quantities is Y = 1/1 as given in 2.2.1. Using, for example, kilometres per hour, metres and seconds as the units for velocity, length and time, respectively, we may derive the following equation between numerical values: IVlkrn,tl = 3~6I~1rnlI4, The number 3,6 which occurs in this equation results from the particular units chosen; with other choices it would generally be different. If in this equation the subscripts indicating the unit symbols are omitted, one obtains an equation between numerical values which is no longer independent of the choice of units and is therefore not recommended for use. If, nev- ertheless, equations between numerical values are used, the units shall be clearly stated in the same context.