In physics, a force is a push or pull on an object. There are four fundamental forces, three of which are involved in keeping stable atoms in one piece and determining how unstable atoms will decay. ----. The electromagnetic force keeps electrons attached to their atom; the strong force holds the protons and the neutrons together in the nucleus; and the weak force governs how atoms decay when they have too many protons or neutrons. The fourth fundamental force, gravity, only becomes apparent with objects much larger than subatomic particles.