1.2: Physical Quantities and Units
We define a physical quantity by how it is measured or by how it was calculated from measured values. It is either something that can be measured, or something that can be calculated from measured quantities. For example, the mass of an object in grams is a physical quantity because it is measured using a scale. The speed of a moving object in meters per second is also a physical quantity because it is based on two measured quantities (distance in meters, and time in seconds).
The fundamental SI units are the kilogram (kg) for mass, the meter (m) for length, the second (s) for time, and the Ampere (A) for electric current. Derived SI units are based on the fundamental SI units. An example is speed, which is length per unit time.
The metric system is a standardized system of units used in most scientific applications. The SI units are based on the metric system. The metric system is based on a series of prefixes that denote factors of ten. We call these factors of ten orders of magnitude. The prefixes tell us the relative magnitude of the measurement with respect to the base unit. Because the metric system is based on these powers of ten, it is a convenient system for describing measurements in science.
Read this text to review these physical quantities and units. Make sure you are familiar with the prefixes, symbols, values, and examples in Table 1.2. We will use them frequently during this course.