How to create underbrace with nothing on top

You want to use a phantom of the subscript.

documentclass{article}

newcommand{emptyunderbrace}[1]{%
  {underbrace{hphantom{scriptstyle#1}}_{#1}}%
}

begin{document}

$emptyunderbrace{vec{F_p} = frac{-{e}^2}{4pivarepsilon_0{vec{r}}^{,2}}frac{vec{r}}{r}}$.

end{document}