  A classical inequality by Gr\"unbaum provides a sharp lower bound for the
ratio $\mathrm{vol}(K^{-})/\mathrm{vol}(K)$, where $K^{-}$ denotes the
intersection of a convex body with non-empty interior $K\subset\mathbb{R}^n$
with a halfspace bounded by a hyperplane $H$ passing through the centroid
$\mathrm{g}(K)$ of $K$.
  In this paper we extend this result to the case in which the hyperplane $H$
passes by any of the points lying in a whole uniparametric family of
$r$-powered centroids associated to $K$ (depending on a real parameter
$r\geq0$), by proving a more general functional result on concave functions.
  The latter result further connects (and allows one to recover) various
inequalities involving the centroid, such as a classical inequality (due to
Minkowski and Radon) that relates the distance of $\mathrm{g}(K)$ to a
supporting hyperplane of $K$, or a result for volume sections of convex bodies
proven independently by Makai Jr. & Martini and Fradelizi.
