Important questions based on NCERT syllabus for Chapter 6 - Work, Energy and Power:
Question-1: When a rocket is air borne , its casing burns up due to friction. The heat energy which is needed is obtained at whose expense? It is the atmosphere or the rocket?
Solution: When the casing burns up due to the friction, the rocket’s mass gets reduced.
As per the law of conservation of energy:
Total energy = kinetic energy + potential energy
= mgh + 12 mv^2
There will be a drop in total energy due to the reduction in the mass of the rocket. Hence, the energy which is needed for the burning of the casing is obtained from rocket.
Question-2: An artificial satellite loses its energy when orbiting the Earth irrespective of how thin the atmosphere is, due to atmospheric resistance. Then, how does the speed of the satellite increases when it approaches closer to Earth?
Solution: The potential energy of the satellite revolving the Earth decreases as it approaches the Earth and since the system’s total energy should remain constant, the kinetic energy increases. Thus, the satellite’s velocity increases. In spite of this, the total energy of the system is reduced by a fraction due to the atmospheric friction.
Question-3: A molecule with a speed of 300 m s-1 hits the wall of the container at an angle 40∘ with the normal and rebounds with the same speed. During the collision is the momentum conserved? Is it an elastic or an inelastic collision?
Solution: The collision is an elastic collision.
Whether the collision is an elastic or an inelastic collision, the momentum gets conserved. The molecule travels at a speed of 300 m/s and strikes the wall and rebounds with the same speed. Thus, the wall’s rebound velocity is zero. During the collision, the total kinetic energy gets conserved.