(i) Heat energy required for burning of casing of rocket comes from the rocket itself. As a result of work done against friction the kinetic energy of rocket continuously deceases and this work against friction reappears as heat energy.
(ii) The gravitational force is a conservative force, hence, work done by the gravitational force over one complete (closed) orbit of comet is zero.
(iii) As an artificial satellite gradually loses its energy due to dissipation against atmospheric resistance, its potential energy decreases rapidly. As a result, kinetic energy of satellite slightly increases i.e. its speed increases progressively.
(iv) In figure, the man carries the mass of 15 kg on his
hands and walks 2 m. In this case, he is actually doing work against the friction force.