Hi, I am learning about SHM and in order to explain why the acceleration is proportional to the displacement but in opposite directions the book used as an example a particle with mass m, attached to a spring. He defines the displacement to the right as being positive and to the left as being negative. When the particle is displaced to the right, x, the extension, will be positive. The tension in the spring is towards the left. But why? Is it a characteristic of the spring to always have its force trying to restore the spring to its equilibrium position? Thanks, Peter G.