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What does |v|/v mean? Wouldn't that always be equal to 1?
v is vector, and has both a magnitude and a direction. |v| is just the magnitude of v (also known as the speed). So, v/|v| would yield a normalized direction vector. In English, that means that v/|v| gives you the direction of the velocity without telling you how fast it's going. This notation can be somewhat confusing for those not familiar with more advanced textbooks, since most introductory textbooks use "hat" notation. For more information, try here: http://en.wikipedia.org/wiki/Unit_vector
Please note that physicists like to write magnitudes like this |u|, while mathematicians like to write it like this ||u||. The reason why we need the direction of velocity at all is because friction opposes motion, so we need our friction to be pointing in the opposite direction of the velocity.
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