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The quantities in bold face are four vectors.

*Hartle pg 88*defines a three force as :

##\frac{d \vec{p}}{d t} \equiv \vec{F}##

Where ##\vec{p}=\frac{m \vec{V}}{\sqrt{1-\vec{V}^2}}## is the three momentum.

Is this three force what we measure? I.e The force that we used in Newtonian mechanics?