1.3 Standards and Units

Physics is an experimental science, and we usually use number to describe the results of measurements. Any number that is used to describe a physical phenomenon quantitatively is called a physical quantity. Some physical quantities are so fundamental that we can define them only by describing how to measure them. Such a definition is called an operational definition.
In other cases we define a physical quantity by describing a way to calculate it from other quantities that we can measure. In the first case we might use a ruler to measure a distance or a stopwatch to measure a time interval. In the second case we might define the average speed of
a moving object as the distance traveled(measured with a ruler) divided by time of travel (measured with a stopwatch).

When we measure a quantity, we always compare it with some reference standard. When we say that a car is 4 meters long, we mean that it is 4 times as long as a meter stick, which we define to be 1 meter long. Such a standard defines a unit of the quantity. The meter is a unit of distance, and the second is a unit of time. when we use a number to describe a physical quantity, we must specify the unit that we are using; to describe a distance simply as "4" wouldn't mean any thing.

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