Force
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Force is what causes an object to undergo rectilinear acceleration, namely to to change its velocity, which in turns is the rate of change of displacement with respect to time. Mathematically, acceleration is the second derivative of displacement. The SI unit for it is the newton. Force is a vector, consisting of a magnitude and direction. The resulting acceleration is in exact linear proportion to the magnitude of the force applied, and in the same direction. The constant of proportionality is known as the coefficient of inertia, or, more commonly, mass. The more mass an object has, the more difficult it is to cause a change in motion.
There are various types of forces, however, all of them are consequences of fundamental forces: gravitational, electromagnetic, strong nuclear and weak nuclear. The strong nuclear force holds neutrons to protons in the nucleus of an atom while the weak nuclear force is responsible for radioactive decay. The electromagnetic and weak nuclear forces have already been united into a single electroweak theory, which governed the time when the Universe was very hot and dense, before the two forces broke symmetry. The strong nuclear force has yet to be united with electroweak in a Grand Unified Theory (or GUT). The holy grail of physics is to unite this GUT with a quantum theory of gravity to produce a Theory of Everything (or TOE). At present, However, the Standard Model of particle physics encompasses what physicists know for certain about the three non-gravitational interactions whereas general relativity is the current theory of gravity.