In science classes, numbers are often used to describe a measurement.  As a result, when we report a number, it almost always needs a unit. Without the unit, it is often difficult (or even impossible) to figure out what a number means.  As an example, if I told you I needed a small job done and promised to pay you 10, how can you decide whether or not to take the job? Certainly, your interest in accepting my offer will be radically different if I’m offering to pay you 10 cents rather than 10 dollars.  But it is not even entirely clear whether I am offering to pay you a lump sum, say 10 dollars for completing the job, or I am describing a rate, like 10 dollars an hour. Because I haven’t given you the units that will go with the number 10, it is not clear what the dimensions of the number are.

With this in mind, there are a handful of skills we need to make sure we are comfortable with when working with units.  For starters, we need to make sure we always report a measurment with its appropriate unit so we can interpret a given value correctly.  If I tell you I need you to travel a distance of 100,000, it matters greatly whether that distance has a unit of millimeters, and is therefore equivalent to 100 meters, or a unit of miles, and requires you to circle the earth roughly four times.  Second, we need to be able to use units to determine the dimensions of a calculated quantity through the process of dimensional analysis.  And lastly, we need to be able to convert from one unit to another.  We will need to convert between units within the same unit system and from units in one unit system to another.