Fundamental Forces in Nature

All the forces observed in nature are ultimately manifestations of just FOUR fundamental interactions.

1. GRAVITATIONAL FORCE: the attractive force between any two point masses separated by a distance, given by $F=\frac{Gm_1m_2}{r^2}$, where $G=6.67\times10^{-11}$ (SI units). It is the weakest of the four fundamental forces for two given point masses at a given separation, but has infinite range and is always attractive; it governs the large-scale structure of the universe. Our everyday experience of it is our own weight, $W=\frac{GMm}{R^2}=mg$, where M and R are the Earth's mass and radius; substituting numbers gives $g=\frac{GM}{R^2}\approx 9.8$ m/s^2. Because everyday objects have masses far smaller than the Earth's, mutual gravitational attraction between ordinary bodies (e.g. two 300 kg sumo wrestlers 0.5 m apart) is utterly negligible compared to their weight due to the Earth.
2. ELECTROMAGNETIC (EM) FORCE: an attractive or repulsive force between electrically charged particles. Electricity and magnetism, once thought independent, were unified into one electromagnetic theory following the work of Faraday (1791-1867) and Maxwell (1831-1879). EM forces are much stronger than gravity and govern nearly all of daily life: friction, normal reaction, tension in strings, collision forces, elastic forces and viscosity are all fundamentally electromagnetic, arising from deformation-induced changes in intermolecular distances.
3. STRONG (NUCLEAR) FORCE: the strongest of the four forces, binding protons and neutrons together inside a nucleus; despite being strongest, it has an extremely short range (less than about $10^{-14}$ m) and is charge-independent.
4. WEAK (NUCLEAR) FORCE: responsible for radioactive decay processes such as beta emission (a neutron converting to a proton, or vice versa, with emission of an electron/positron and a neutrino); it is stronger than gravity but much weaker than the strong and EM forces, with an even shorter range (of order $10^{-16}$ m) than the strong force. A key clue that a weak-force process has occurred is the emission of a neutrino. …