We throw a ball. We sit on a chair. We keep contact forces around to persecute you in this course. We saw one example of a contact force in the last lecture, when we talked about action and reaction in a mass sitting on a table. The table pushes up on the mass to keep it from accelerating downward under the influence of gravity.
Contact forces are normally macroscopic manifestations of one of the four fundamental forces.
In physics, a spring is an object that responds in a particular way discussed below. It is also massless, in physics. The minus sign means that the force is always directed in such a way as to accelerate the object toward the rest position xo. The fact that the force depends linearly upon the compression of extension is often observed in real springs, at least for small displacements. The dynamics of the protein is determined by these springs.