= Weak viscous torque in a thin-disk plunging region
{title2=$\mathcal G_{\rm plunge}/\mathcal G_{\rm circular}=O[\alpha(H/r)^2]$}
A <thin disk> with <alpha disk> viscosity $\nu\sim\alpha H^2\Omega$ has a <viscous timescale> much longer than its dynamical infall time in the <plunging region of a black-hole accretion disk>. At fixed nonzero <accretion rate>, rapid inflow reduces its <surface density of a disk>. The resulting <viscous torque in an accretion disk> is smaller by the displayed factor than in the slowly drifting circular disk. This estimate concerns radii outside the narrow matching layers.
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