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Comets move around the sun in highly e...

Comets move around the sun in highly elliptical orbits . The gravitational force on the comet due to the sun is not normal to the comet's velocity in general. yet the work done by the gravitational force over energy complete orbit of the comet is zero . Why ?

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For a conservative force work done over a path is minus of change in potential energy. Over a complete orbit, there is no change in potential energy.
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