# 1.4 Units of Measurement

The scientific community uses SI units for measurement of such properties as mass, length, and temperature. There are seven SI base units from which all other necessary units are derived.

Although the meter is the base SI unit used for length, it may not be convenient to report the length of an extremely small object or an extremely large object in units of meters. Decimal prefixes allow us to choose a unit that is appropriate to the quantity being measured. Thus, a very small object might best be measured in millimeters (1 millimeter = 0.001 meters), while a large distance might best be measured in kilometers (1 kilometer = 1000 meters).

The SI unit of temperature is the kelvin, although the Celsius scale is also commonly used. The Kelvin scale is known as the absolute temperature scale, with 0 K being the lowest theoretically attainable temperature.

K = ºC + 273.15

Figure 1.18 shows a comparison of the Kelvin, Celsius, and Fahrenheit scales.

 Figure 1.18.

Note that there are no units of volume in Table 1.4. For measurements of volume, density, and other properties, we must derive the desired units from SI base units. In the case of volume, which has units of length cubed, (length)3, the basic SI unit for volume is the cubic meter (m3). This is an extremely large volume, though, and more often you will see volumes reported in liters, L (1 cubic decimeter, or 1 dm3), or milliliters, mL (which are the same as cubic centimeters: 1 mL = 1 cm3).

Density has units of mass per unit volume and is often reported as grams per cubic centimeter, g/cm3.

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