By examining the effective potential energy find the radius at which a planet (or cornet) with angular momentum t can orbit the sun in a circular orbit with fixed radius. [look at du_eff/dr.] (b) show that this circular orbit is stable, in the sense that a small radial nudge will cause only small radial oscillations. [look at d^2u_eff/dr^2.] show that the period of these oscillations is equal to the planet's orbital period. u_eff(r) = - g m_1 m_2/r + l^2/2 mu r^2.