Four-force

F^a:= {dP^a \over d	au}

.

Since

P^a = m_0U^a

where ''m''₀ is the Rest Mass and ''U^a'' is the Four-velocity , we can relate the four-force with the Four-acceleration as like Newton's Second Law :

:

F^a = m_0A^a = \left(\gamma\dot mc,\gamma\mathbf f
ight)

.

Here,

m = m_0\gamma

\mathbf f=m_0\left(\dot\gamma\mathbf u+\gamma\mathbf{\dot u}
ight)

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