ABSTRACT
How tall are you? You could answer 5 feet 8 inches, or 68 inches, or 1.73 meters. These are, of course, all the same heights, but the numbers are different because they are given in different units of length. Since science, in general, and physics, in particular, deal with measurement, in either an experiment or a prediction of an experimental outcome, we want to clearly de ne the units we will be using. Not only will this be important for clarity, but it can have very important practical consequences. Some of you may have heard that a scienti c experiment failed several years ago because manufacturers of different parts of the apparatus used different systems of units while each one assumed the other was using the same units as they were.
