SHM --> F = -kx SHM assumes that F is the only force acting on the system, so if we have a mass held between two springs on a linear air track, the F = -kx force refers to the restoring force from the springs? Is that the only thing it refers to? What about air resistance as the gliding object travels horizontally along the air track? I've noticed also that the weight of the gliding object introduces friction so it's not exactly a closed system, could I explain this by saying that this is because F = -kx which acts horizontally, is perpendicular to the force of the weight? So F = -kx does not accoutn for the friction introduced by weight? Or am I wrong to say that becase the friction introduced by the weight is acting horizontally as well? I'm confounded.