I was surprised one day to hear the instructor in an introductory physics class claim that "memorization is useless." He meant that it won't help you succeed in a physics class. Now this professor is a smart guy, but this claim is untrue. If he'd qualified it by saying that memorization is not enough, that would be different. Certainly it's true that compared with a history class, remembering random facts is a relatively unimportant skill in physics. But he didn't say that, so his actual statement, that "memorization is useless", is nonsense.

The professor tried to support his claim by showing how, if he happened to forget the quadratic formula, he could quickly derive it. That's fine, but you have to start from somewhere, and the more you know, i.e. the more you remember, the less work you have to do.

Let's face it. You sit down to write a typical physics exam and you have 50 minutes to solve 3 to 5 problems. You have to be fast. If you can avoid it, you don't want to spend time deriving equations. Even time referring to a sheet of notes, if you're allowed one, is time wasted. If you just KNOW the kinematic equations, for example, you save time. And if you have more time, you probably end up with a better grade.

I'm not advocating memorizing every equation in the book, but you should know key definitions and the equations you need most often. There aren't as many as you might think. Of the page after page of equations in the book, which ones are the key ones? Well, which ones have you actually used in solving homework problems (or in solving extra practice problems, or problems that were given on previous exams)?

You shouldn't actually spend much time explicitly memorizing. It should tend to happen naturally, as you work with the basic concepts. If you understand the concept well enough, you can just write it down in the form of an equation. If you understand the concept, there's little extra memorization required. Though memorization isn't everything, it's certainly not useless. Remember that.