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Question -

 Answer thefollowing:
(a) The casing of a rocket inflight bums up due to friction. At whose expense is the heat energy requiredfor burning obtained? The rocket or the atmosphere?
(b) Comets move around the sun inhighly elliptical orbits. The gravitational force on the comet due to the sunis not normal to the comet’s velocity in general. Yet the work done by thegravitational force over every complete orbit of the comet is zero. Why?
(c) An artificial satelliteorbiting the earth in very thin atomosphere loses its energy gradually due todissipation against atmospheric resistance, however small. Why then does itsspeed increase progressively as it comes closer and closer to the earth?
(d) In Fig.(i), the man walks 2 mcarrying a mass of 15 kg on his hands. In Fig. (ii), he walks the same distancepulling the rope behind him. The rope goes over a pulley, and a mass of 15 kghangs at its other end. In which case is the work done greater?



Answer -

(a) Heat energy required for burning of casing ofrocket comes from the rocket itself. As
a result of work done against friction the kinetic energy of rocketcontinuously decreases – and this work against friction reappears as heatenergy.
(b) This is because gravitational force is a conservative force. Work done bythe gravitational ‘ force of the sun over a closed path in every complete orbitof the comet is zero.
(c)  As an artificial satellite gradually loses its energy due todissipation against atmospheric resistance, its potential decreases rapidly. Asa result, kinetic energy of
satellite slightly increases i.e., its speed increases progressively.
(d) In Fig. (i), force is applied on the mass, by the man in vertically upwarddirection but distance is moved along the horizontal.
θ = 90°. W = Fs cos90° = zero
In Fig. (ii), force is applied along the horizontal and the distance moved isalso along the horizontal. Therefore, θ = 0°.
W = Fs cos θ =mg x s cos 0°
W = 15 x 9.8 x 2 x 1 = 294 joule.
Thus, work done in (ii) case is greater.

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