I'm making a vibration rig that represents a classic 2 mass, 3 spring system (See attached Image). (To clarify the image - there will be no forcing frequency. m=m1=m2 and k=k1=k2=k3) I'm using extension springs in the vibration rig. I plan to displace them in extension at an equilibrium point that will have them remain in tension throughout harmonic motion. The springs will therefore always be in tension and will always only exert a tension spring force on the mass. My question is: In the equations of motion do we always assume the spring constant will exert force in both tension and compression? In other words, If compressed it pushes away and if stretched it pulls back. If so, how will this affect the system I've proposed above? A system where closing the spring it is still in tension and not opposing the motion. Thanks for taking the time to read this. Any help is appreciated.