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Based on such considerations, the International Standards Organization recommends using seven base quantities, which form the International System of Quantities (ISQ). These are the base quantities used to define the SI base units. [link] lists these seven ISQ base quantities and the corresponding SI base units.
ISQ Base Quantity | SI Base Unit |
---|---|
Length | meter (m) |
Mass | kilogram (kg) |
Time | second (s) |
Electrical current | ampere (A) |
Thermodynamic temperature | kelvin (K) |
Amount of substance | mole (mol) |
Luminous intensity | candela (cd) |
You are probably already familiar with some derived quantities that can be formed from the base quantities in [link] . For example, the geometric concept of area is always calculated as the product of two lengths. Thus, area is a derived quantity that can be expressed in terms of SI base units using square meters $(\text{m}\phantom{\rule{0.2em}{0ex}}\times \phantom{\rule{0.2em}{0ex}}\text{m}={\text{m}}^{2}).$ Similarly, volume is a derived quantity that can be expressed in cubic meters $({\text{m}}^{3}).$ Speed is length per time; so in terms of SI base units, we could measure it in meters per second (m/s). Volume mass density (or just density) is mass per volume, which is expressed in terms of SI base units such as kilograms per cubic meter (kg/m ^{3} ). Angles can also be thought of as derived quantities because they can be defined as the ratio of the arc length subtended by two radii of a circle to the radius of the circle. This is how the radian is defined. Depending on your background and interests, you may be able to come up with other derived quantities, such as the mass flow rate (kg/s) or volume flow rate (m ^{3} /s) of a fluid, electric charge $(\text{A}\xb7\text{s}),$ mass flux density $\text{[kg/}({\text{m}}^{2}\xb7\text{s)],}$ and so on. We will see many more examples throughout this text. For now, the point is that every physical quantity can be derived from the seven base quantities in [link] , and the units of every physical quantity can be derived from the seven SI base units.
For the most part, we use SI units in this text. Non-SI units are used in a few applications in which they are in very common use, such as the measurement of temperature in degrees Celsius $(\text{\xb0}\text{C}),$ the measurement of fluid volume in liters (L), and the measurement of energies of elementary particles in electron-volts (eV). Whenever non-SI units are discussed, they are tied to SI units through conversions. For example, 1 L is ${10}^{\mathrm{-3}}{\phantom{\rule{0.2em}{0ex}}\text{m}}^{3}.$
Check out a comprehensive source of information on SI units at the National Institute of Standards and Technology (NIST) Reference on Constants, Units, and Uncertainty.
The initial chapters in this textbook are concerned with mechanics, fluids, and waves. In these subjects all pertinent physical quantities can be expressed in terms of the base units of length, mass, and time. Therefore, we now turn to a discussion of these three base units, leaving discussion of the others until they are needed later.
The SI unit for time, the second (abbreviated s), has a long history. For many years it was defined as 1/86,400 of a mean solar day. More recently, a new standard was adopted to gain greater accuracy and to define the second in terms of a nonvarying or constant physical phenomenon (because the solar day is getting longer as a result of the very gradual slowing of Earth’s rotation). Cesium atoms can be made to vibrate in a very steady way, and these vibrations can be readily observed and counted. In 1967, the second was redefined as the time required for 9,192,631,770 of these vibrations to occur ( [link] ). Note that this may seem like more precision than you would ever need, but it isn’t—GPSs rely on the precision of atomic clocks to be able to give you turn-by-turn directions on the surface of Earth, far from the satellites broadcasting their location.
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