[10/12, 6:54 PM] Anshuman: Q10. An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down. Q11. What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer. When a satellite moves round the Earth, then at each point of its path, the direction of force of gravity on the satellite (along the radius) is perpendicular to the direction of its displacement (along the tangent). Hence, the work done on the satellite by the force of gravity is zero. Q12. Can there be displacement of an object in the absence of any force acting on it? Think. Discuss this question with your friends and teacher. The answer is both Yes and No. Yes because when an object moves in deep space from one point to another point in a straight line, the displacement takes place, without the application of force. No, because force cannot be zero for displacement on the surface of earth. Some force is essential. [10/12, 6:55 PM] Anshuman: Q13. A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer. The person does not do work because no displacement takes place in the direction of applied force as the force acts in the vertically upward direction. Q14. An electric heater is rated 1500 W. How much energy does it use in 10 hours? Energy consumed by an electric heater can be obtained with the help of the expression, P=W/t where, Power rating of the heater, P = 1500 W = 1.5 kW Time for which the heater has operated, t = 10 h Work done = Energy consumed by the heater Therefore, energy consumed = Power × Time = 1.5 × 10 = 15 kWh Hence, the energy consumed by the heater in 10 h is 15 kWh or 15 units. Q15. Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy? When the pendulum bob is pulled (say towards left), the energy supplied is stored in it is the form Of PE on account of its higher position. When the pendulum is released so that it starts moving towards right, then its PE changes into KE such that in mean position, it has maximum KE, and Zero PE. As the pendulum moves towards extreme right, its KE changes into PE such that at the extreme position, it has maximum PE and zero KE. When it moves from this extreme position to mean position, its PE again changes to KE. This illustrates the law Of conservation of energy. Eventually, the bob comes to rest, because during each oscillation a part of the energy possessed by it transferred to air and in overcoming friction at the point of suspension. Thus, the energy of the pendulum is dissipated in air. The law of conservation of energy is not violated because the energy merely changes its form and is not destroyed. Q16. An object of mass, m is moving with a constant velocity, v. How much work should be done on the object in order to bring the object to rest? Kinetic energy of an object of mass m moving with a velocity v is given by the expression 1/2mv². To bring the object to rest, an equal amount of work i.e. 1/2mv² is required to be done on the object.
