# Energy in orbital motion

Hey, so I have a question about motions of planets and their energy basically.

When we have a circular orbit, why is it that the kinetic energy is just the opposite of potential energy? (Assuming it's a closed orbit)

Like if we have U = something, than the kinetic energy T = -1/2U? This would be saying the kinetic energy doesn't change for a circular orbit but the potential energy does, and than I would think this would be a parabolic orbit as energy would then equal to 0 and epsilon (eccentricity) is equal to 1.

I hope that made sense, I'm having trouble understanding such motion.

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Simon Bridge
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When we have a circular orbit, why is it that the kinetic energy is just the opposite of potential energy? (Assuming it's a closed orbit)
Have you followed the derivation?
http://www.pha.jhu.edu/~broholm/l24/node1.html [Broken]

Like if we have U = something, than the kinetic energy T = -1/2U? This would be saying the kinetic energy doesn't change for a circular orbit but the potential energy does...
No - if U changes, the T will also change. If U does not change, then neither does T.
Note: that should be T=-(1/2)U

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Interesting, because recently I did a problem, for which the kinetic energy remained the same and the potential energy had changed, so that is where most of the confusion comes from.

Simon Bridge