1. The problem statement, all variables and given/known data So there are 4 masses in a line. They are connected by 3 springs between them, denoted by k_12 & l_12 (spring coefficient + natural extension length), k_23 & l_23, etc. 2. Relevant equations Write down the f = ma equations of motion for all 4 masses. 3. The attempt at a solution See my image. This problem really annoys me because I always learn how to do it for the test but never remember. I think I've gone through 3 courses where this problem came up and every time I could never remember. Well now it's just a random exercise in a linear algebra text I'm reading and I wouldn't mind learning it right once and for all. After all I'm supposed to be an engineer. Thank you for any help.