= Equal-mass Coulomb circular orbit
{title2=$T=2\pi\sqrt{ms^3/(2k_eq^2)}$}
For two particles of equal <mass> $m$ and opposite charges $\pm q$ separated by $s$, the <centre of mass> lies halfway between them. In a nonradiating classical model, each radius is $s/2$ and $k_eq^2/s^2=m\omega^2s/2$. Hence $\omega^2=2k_eq^2/(ms^3)$ and $T=2\pi\sqrt{ms^3/(2k_eq^2)}$. Equivalently, relative motion has <reduced mass> $m/2$. Radiation and quantum binding invalidate an indefinitely stable classical orbit for actual microscopic charges.
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