The Four-force reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Four-force

In the special theory of relativity four-force is a four-vector that replaces the classical force; the four-force of the four-vector a is defined as the change in four-momentum over the particle's own time:

In four-vectors formalism the electromagnetic field is described by a second-order tensor (which is written as a 4 on 4 matrix).

See also: four-vector, four-velocity, four-acceleration, four-momentum.

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