I'm a freshman in physics at Notre Dame. What I have noticed is that no one knowledgeable in physics actually explains the principles. Instead, as an example,my professor will say something like, "And so, this allows you to derive this equation." But he doesn't ever explain the "why" behind the equations, so I never know exactly when to apply them. Is there any help site online that actually explains the principles and why one would use them? Thanks in advance.