Pure force
= Pure force
{title2=$F\cdot U=0$}
A <four-force> preserving the particle's <rest mass>. For a massive particle, $P=mU$ and $U^2=c^2$ in the mostly-minus convention imply $F\cdot U=c^2dm/d\tau$. Thus constant mass is equivalent to orthogonality of the force and <four-velocity>. <Energy> transfer in a moving frame is allowed even though <rest mass> does not change.