  Based on the instantaneous Bethe-Salpeter equation method, we calculate the
average values $\overline{|\vec{q}|^n}\equiv q^n$ and speed powers
$\overline{|\vec{v}|^n} \equiv v^n$ ($n=1,2,3,4$) of a heavy quark inside $S$
wave and $P$ wave heavy-light mesons, where $\vec{q}$ and $\vec{v}$ are the
three dimensional momentum and velocity of the heavy quark, respectively. We
obtain the kinetic energy $\mu^2_{_\pi}=0.455$ GeV$^2$ for the $B$ meson, which
is consistent with the experimental result $0.464\pm 0.076$ GeV$^2$. For the
$B_{s}$, $D$ and $D_{s}$ mesons, the $\mu^2_{_\pi}$ are $0.530$ GeV$^2$,
$0.317$ GeV$^2$ and $0.369$ GeV$^2$, respectively. And $v^2=0.0185$, $0.0215$,
$0.121$, and $0.140$ for $B$, $B_{s}$, $D$, and $D_{s}$. We obtain some
relationships, for example, $q^n_{_{0^-}}(mS)\approx q^n_{_{1^-}}(mS)$,
$q^n_{_{0^+}}(mP)\approx q^n_{_{1^{+'}}}(mP^{'})> q^n_{_{1^+}}(mP)\approx
q^n_{_{2^+}}(mP)$, and $q^n(mS)< q^n(mP)$ ($m=1,2,3$), etc.
