One of you had a good question about the difference in how I presented
Hooke's Law (F = -kx) and how the book does it (F = ks). The
difference is that in the book (p. 408), they think of F as the force
required ON the spring the make it deform, whereas I think of F as the
force exerted BY the spring once it is deformed (i.e., the restoring
force). The difference is simply the direction, a minus sign (the
magnitude is the same). Really, F is a vector with direction, as is x,
but I left the vector signs off in the equation. Personally I think it
is more helpful to think of them as vector opposites, since this comes
in handy when you are doing problems and need to draw force diagrams
and then balance forces, etc.