http://www.ck12.org Chapter 6. Newtons Laws Version 2
6.3 What are Forces?
In other words, things tend to stay in the their current state of motion unless some "forces" are "impressed" on them.
But where do such forces come from? Whatare they? Force isn’t a realobject, but rather aconceptused to
describe actions. We can think of it as the cause of any kind of "pushing" or "pulling" that an object experiences.
As long as we can measure them consistently, forces can be treated like any other physical vector quantity. One way
to state a major goal of physics is to find a method for consistently predicting the forces an object will experience
under any circumstances, based on the circumstances. At this point, physicists have identified four basic forces that
govern the universe:
- The strong force: The most powerful of the four forces, it holds nuclei together in atoms — but has a very
short range. It has to overcome the massive electromagnetic repulsion between protons in a nucleus. - The electromagnetic force: Responsible for the behavior of charged particles. Has infinite range.
- The weak force: Another nuclear force, responsible for much of the structure of stars and the universe in
general. Its range is longer than that of the strong force, but still smaller than an atom. - Gravity: Responsible for the attraction of all masses in the universe. Has infinite range.
All others — friction, air resistance, and other contact forces; buoyancy; the spring force — can be reduced to these
fundamental forces. The fundamental forces are covered in more detail in later chapters.
Note that this classification does not tell us anything aboutwherethese forces come from, or how they are able to
act seemingly at a distance, with "no strings attached". Newton himself said:
I have not as yet been able to discover the reason for these properties of gravity from phenomena, and I do not feign
hypotheses.
In other words, we are interested in describing and predicting nature, rather than explaining its root causes. Forces
acting a distance may seem strange in light of our experience with forces thatwecan apply to things (like pushing
on rocks, etc), but to Newton asking about the nature of gravity was like asking about the nature of mass: it’s just
there, we can measure it, and that’s it.