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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?

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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The casing of a rocket in flight burns up due to friction. At whose expense is the heat energy required for burning obtained– the rocket or the atmosphere?

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?

The sign of work done by a force on a body is important to understand. State carefully if the following quantities are positive or negative,

(e) Work done by the resistive force of air on a vibrating pendulum in bringing it to rest.

Given in Fig. 6.11 are examples of some potential energy functions in one dimension. The total energy of the particle is indicated by a cross on the ordinate axis. In each case, specify the regions, if any, in which the particle cannot be found for the given energy. Also, indicate the minimum total energy the particle must have in each case. Think of simple physical contexts for which these potential energy shapes are relevant.

Work, Energy and Power

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?