It's a non-trivial point though. As you'll learn eventually, when solving differential equations the obtained values are often positive, even when the "correct" answer could be positive or negative. Therefore, you have to phrase the whole thing in terms of absolute values. Learning high school arithmetic is painfully boring, but it becomes useful eventually. So far, no one has found a way to make it not boring from the start. FYI, what kb is referring to above in the text you quoted is the Harmonic Oscillator, which is the most important solution to Newton's second law (although you'll learn nothing of it until calc-based physics). It basically rules the world.See, this is what I'm talkin' about! Where the hell was this when I spent a month learning that absolute values can't be negative?