1. The problem statement, all variables and given/known data I have trouble determining the system of forces in systems with strings like the following: 2. Relevant equations 3. The attempt at a solution My guess is it is: x1=L/3 x2=2L/3 mass 1: -kx1+k(x2-x1)=m1a1 mass 2: -k(x2-x1)+kx1=m2a2 I'm not sure it is correct. Even if it is, anyone knows a good way to see the correct signs of the forces? I have some trouble identifying them... Thanks ahead! D.