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FORCE

Course of change of momentum with time. Forces are said to reason accelerations via f = ma (Newton's law).
There are four primary forces known presently: the gravitational, electromagnetic, weak nuclear, and strong nuclear forces. The gravitational and electromagnetic forces are long-range (dropping as 1/distance^2), while the nuclear forces are shortrange (effective only within nuclei, distances on the order of 10^-15 metres). The electromagneticforce is many stronger than the gravitational force, but is usually cancelled over big distances because of the balance of positive and negative charges.
See: momentum.