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Comets mover 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 orbit of the comet is zero. Why?

(a) The gravitational force is conservative, hence work done is zero

(b) The gravitational force is non-conservative, hence work done is zero

(c) Energy is absent, hence work done is zero

(d) Force is in negative direction

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I would like to ask this question from Coefficient of Restitution topic in division Gravitation of Engineering Physics I

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The correct choice is (a) The gravitational force is conservative, hence work done is zero

Explanation: The gravitational force acting on the comet is a conservative force. The work done by a conservative force is equal to the negative of the change in potential energy. Over a complete orbit of any shape, there is no change in potential energy of the comet. Hence no work is done by the gravitational force on the comet.

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