Barycentric frame কী?

Example 19.6 (A rolling wheel)

A wheel of mass $M$, radius $R$, moment of inertia $J$ about its axle, rolling without slipping at speed $v$: the contact point is at rest, so $\omega = v/R$, and $E_k = \tfrac12 Mv^2 + \tfrac12 J\omega^2 = \tfrac12 Mv^2(1 + J/MR^2)$ — three quarters of $Mv^2$ for a uniform disk ($J = \tfrac12 MR^2$), $Mv^2$ for a hoop. The second term is why a bicycle’s wheels are made light: their rotation costs energy that the frame’s mass does not.