I find it somehow confusing to imagine why m*a=-k*x is "generally valid". that minus sign bothers me. Imagine I raised a bob to some height (ex. from the left side) and then released it. from that moment till the moment where it reaches equilibrium position m*a and restoring force have same direction. from a moment where it "goes of" the equilibrium position to the moment when it reaches the same height on the right side m*a and restoring force have different direction and so on. should it be m*a=+-k*x? what I'm missing?