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fundamental forces Four basic forces that exist in physics. The most familiar, and the weakest, is gravitation. The gravitational force between the Earth and an object accounts for the object's weight. Much stronger is the electromagnetic force, which ‘binds’ particles together. The two other forces operate only on the subatomic level: the weak nuclear force, associated with the decay of particles, is intermediate in strength between the gravitational and electromagnetic force; the strong nuclear force, associated with the ‘glue’ that holds nuclei together, is the strongest natural force.