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 P: 615 The Energy of a Multiparticle System Don't worry, the virial theorm is part of action based lagrangian mechanics, you probably haven't encountered it. You should use the fact that the orbit is circular, that is to say that r doesn't change. So try playing about with the equation for the inwards force and the outwards centripital (centrifugal? I can't never remember which is the outwards pushing one) $a_{outwards} = \frac{v^2}{r}$ $a_{outwards} + a_{inwards} = 0$ See if you can work out the kinetic energy from that then see what happens when you compare it to the potential energy