There are at present only four certainly known types of force, and these are listed below. The relative importance of each force in an interaction depends on the type of interaction being considered.
(a) The gravitational force acts between all particles with mass and is responsible for holding planets in orbit around the Sun. Range – infinite varying as 1/d2.
(b) The electromagnetic force acts between all charged particles, and is the binding force of atoms and molecules. Range – infinite varying as 1/d2
(c) The weak force is responsible for radioactive decay and the change in quark flavour. It acts between all particles. It is seen in lepton reactions such as the reaction between a neutrino and a muon. Range – about 10-3 fm
(d) The strong force holds neutrons and protons together in a nucleus. It acts between hadrons since they contain quarks. Range – about 1 fm.
Note: 1 fm = 1 fermi = 10-15 m about the diameter of a small atomic nucleus.