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Q5.5: NCERT Physics - Part 1 WORK, ENERGY AND POWER 5.5 Solution: Answer the following: (a) The casing of a rocket in flight…
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? (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. 5.13(i) the man walks 2 m carrying a mass of 15 kg on his hands. In Fig. 5.13(ii), he walks the same distance pulling the rope behind him. The rope goes over a pulley, and a mass of 15 kg hangs at its other end. In which case is the work done greater?
Step-by-Step Verified Solution
**Answer:** (a) At the expense of the rocket's kinetic energy. (b) Gravitational force is conservative. (c) Loss of potential energy exceeds loss of kinetic energy. (d) Work is greater in case (ii).
**Step-by-Step Explanation:**
(a) The heat energy comes from the kinetic energy of the rocket. Friction converts kinetic energy into heat, causing the casing to burn.
(b) The gravitational force is conservative. The work done by a conservative force over a closed path (complete orbit) is zero.
(c) As the satellite comes closer, its potential energy decreases (becomes more negative). The loss in potential energy is greater than the loss in total energy, so kinetic energy (and hence speed) increases.
(d) In case (i), the force is vertical (supporting the mass) while displacement is horizontal, so work done by the man is zero. In case (ii), the force is along the rope and has a component in the direction of displacement, so work done is positive and greater.
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