I am studying the problem of two masses, m1 and m2 (not equal) attached by a spring of spring constant k on a frictionless surface set into oscillatory motion (unstretched spring is of length l, x1 < x2). When I first attacked this problem, I let the origin be the center of mass, calculated the reduced mass, and was able to get the frequency, etc. While this yielded the correct answer, I have now been asked to solve it setting up the equations of motion. I deduce that they are m1a1=-k(x1-x2+l) m2a2=-k(x2-x1-l) Can someone help me understand the notion of l? That is, for mass 1 is l positive because it is in the positive direction and vice-versa? I can solve it mathematically from there, but it makes me uncomfortable not fully understanding how the equations got set up. Thanks in advance!