2. Measurement Many properties of matter are quantitative, that is, we can write their value as a number and a unit. 2.1. The International System of Units (SI) To measure a quantity, for example, length , we need a unit. Look at this brother and sister : Using dif ferent units makes it dif ficult to compare values. For this reason , an international organisation , the General Conference on Weights and Measures, established a set of units called the International System of Units (SI). Many countries, including Spain , use it. To give ver y large or ver y small quantities, the SI also established multiples and submultiples of these units. 11 Imagine you are going to design some packaging. What quantities do you need to know about the material you want to package? How can you determine the value of these quantities? 12 Can you design a large package that can contain various individual packages? How would you do this? What are the advantages and disadvantages of large packages compared to small packages? We give the result of the measurement as a number followed by the unit used: length of the batter y = 5.0 cm. The International System of Units (SI) defines the symbols for units, multiples and submultiples, as well as the rules for writing them. The symbol for units is written in lower case unless it refers to the name of a person : m (metre), J ( joule). The symbol for multiples and submultiples is written before the unit: km, cL. Symbols never take the plural "s". Therefore, we write eight kilometres as 8 km, not 8 kms. A quantity is any property of matter that can be measured . It is written with a number (the amount) and a unit. A unit is a known amount of a quantity that we use as a reference to measure that quantity. To measure a quantity, we compare it with a unit to establish how many multiples of that unit it contains. Length of the battery 5.0 cm quantity amount unit His younger sister measures it in steps: it is six steps long. The older brother measures the rug in palms: it is 14 palms long. What is the measurement of the rug? CHALLENGE 14
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