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In physics (not to mention the rest of science, not to mention the rest of the world...) the unit of distance is the meter. It will often be convenient for us to measure smaller distances in centimeters or millimeters, and larger distances in kilometers. Of course, there are other metric (SI) length units, but these are the ones that you need to be familiar with for this course.
Customary units, such as feet, inches, and miles, are commonly used in a few technologically backward countries of the world, and we will use them from time to time in this course since you are probably most familiar with them. Scientific work, however, is done exclusively in the metric system.
For motion in one dimension, it is usually most convenient to indicate position by choosing a convenient zero position, marking one direction from zero as positive positions, and the other direction as negative positions - number-line style.
It is important to realize that you are free to choose any convenient point as the zero position for your motion (often the starting position is the most convenient), and choose either direction from zero as the positive direction.