question_answer 5)

Answer the following :                             (a) 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 or both ?                                     [Delhi 12] (b) 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 ?                 (c) An artificial satellite orbiting the earth in very thin atmosphere loses its energy gradually due to dissipation against atmospheric resistance, however small. Why then does its speed increase progressively as it comes closer and closer to the earth ?                       (d) In Fig. 6.48 (i) the man walks 2 m carrying a mass of 15 kg on his hands. In Fig. 6.48 (ii), he walks the same distance pulling the rope behind him. The rope goes over a pulley, and a mass of 15kg hangs at its other end. In which case is the work done greater ?                Fig. 6.48                                          

Answer:

(a) Heat energy required for the burning of the casing of a rocket in flight is obtained from the rocket itself. It is obtained at the expense of the mass of the rocket and its kinetic and potential energies. (b) The gravitational force acting on the comet is a conservative force. The work done by a conservative over any path is equal to the negative of the change in P.E. Over a complete orbit of any shape, there is no change in P.E. of the comet. Hence no work is done by the gravitational force on the comet. (c) As the satellite comes closer to the earth, its potential energy decreases. As the sum of kinetic and potential energy remains constant, the kinetic energy and velocity of the satellite increase. But the total energy of the satellite goes on decreasing due to the loss of energy against friction.                                           (d) In case (i), no work is done against gravity because he displacement of 2 m (horizontal) and the weight of 15 kg (acting vertically downwards) are perpendicular to each other. Work is done only against friction.            In case (ii), work has to be done against gravity  in addition to the work to be lone against friction while moving a distance of 2 m. Thus   lie work done in case (ii) is greater than that in case (i).