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Work, Energy and Power
Question
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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 every complete orbit of the comet is zero. Why?

Answer

The gravitational force is conservative force.
We know that, work done by conservative force is independent of path followed, but depends upon initial and final position. Therefore, over the closed path, the work done by conservative force is zero. 

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