Since the fourvelocity is a unit fourvector, the length of the fourmomentum is equal to the mass.
In reactions between an isolated handful of particles, fourmomentum is conserved[?]. The mass of a system of particles may be more than the sum of the particle's masses, since kinetic energy counts as mass. As an example, two particles with the fourmomentums {5, 4, 0, 0} and {5, 4, 0, 0} both have the mass 3, but their total mass is 10. (Note that the length of the fourvector {t, x, y, z} is √(t^{2}x^{2}y^{2}z^{2}).)
The scalar product of a fourmomentum and the corresponding fouracceleration is always 0.
See also: fourvector, fourvelocity, fouracceleration, fourforce.
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