HCI Worksheet 10 Work Energy and Power (answers)

Hwa Chong Institution Sec 3 1 Name: ________________________________________ Class: __________ Date: ___________ Sec 3 Physics Worksheet 10: Work, energy and power SECTION A: CONCEPTUAL QUESTIONS 1 How does friction produce internal energy? Explain what is happening at the molecular level to produce this form of energy. When the surface molecules 'rubbed' against one another, they are displaced from their equilibrium positions and subsequently oscillate strongly. This work done by friction is transferred to a molecular level and is manifested as internal energy (thermal energy) of the molecules. 2 Can the total work done on an object during a displacement be negative? Explain. If the total work is negative, can its magnitude be greater than the initial kinetic energy of the object? Explain. Yes, the work done can be negative. This happens when the direction of the applied force is opposite that of the direction of displacement. e.g. during the slowing down of a car, the displacement is forward while the acceleration (force) is backward. It is important to note that work done on a system is the amount of energy transferred to the system by an external force. If the system speeds up as a result, then positive work is done on the system. If the system slows down, then negative work is done on the system. Care should be taken to distinguish the fact that negative work is different from, say, negative velocity because, unlike velocity, work is a scalar quantity. There are no spatial directions involved. As for the second question, consider this: when the negative work is equal to the positive initial kinetic energy, the object stops. At this point, the external force would have slowed down the object to a complete halt. Further application of the external force will result in positive work d one because the object will now move in the same direction as the force. Therefore, the negative work magnitude can't be greater than the initial kinetic energy. 3 An elevator is hoisted by its cables at a constant speed. Is the total work done on the elevator positive, negative or zero? Explain. Since the elevator is moving at a constant speed, the resultant force on the elevator is zero , and hence the total work done on the elevator is zero. You can think of it as the positive work done by the tension in the cables is negated by the

Hwa Chong Institution Sec 3 2 negative work done by gravity, and the resultant work is zero. ( the tension in the cable is stuffing energy into the elevator while the gravity is taking energy out at the same rate.) 4 A rope tied to an object is pulled, causing the object to accelerate. According to Newton 's Third Law, the object pulls back on the rope with equal and opposite force. Is the net work done on the object zero? Is there a change to the kinetic energy of the object? No, there is non-zero net work done on the object. (Recall that the third law says that the two forces act on different objects; the rope acts on the object while the object acts on the rope , and so the two forces do not act on the same body, and they cannot cancel each other.) There is a change in the kinetic energy as the object is accelerating. This is consistent with the work-energy theorem as the work done by the force is changed into the object's kinetic energy. 5 When you use a jack to lift a car, the force you exert on the jack is much less than the car's weight. Does this mean that less work is done on the car than if the car is lifted directly? While the applied force is smaller than the weight, the work done (applied energy) cannot be smaller than the gravitational potential energy. This idea is discussed in the section on machines. In the ideal case, the amount of work required to lift the car is equal to the gain in GPE. In reality, the amount of work will be higher than GPE because part of the work done is used to overcome resistive forces (friction). 6 A compressed spring is clamped in its compressed position and is then dissolved in acid. What becomes of the spring's elastic potential energy? The elastic potential energy is dissipated in the form of thermal energy in the acid. In other words, dissolving a compressed spring in acid will make the resulting solution slightly hotter than dissolving an identical spring in the relaxed state. No amount of energy is lost in the process (conservation of energy). 7 You bounce on a trampoline, going a little higher with each bounce. Explain how you increase your total mechanical energy. If you drop a stone on the trampoline, you will find that the stone's rebound height will decrease after each rebound. This is expected as resistive forces present in the system will remove energy from the system. A man can increase his height after each rebound because the man is supplying mechanical energy into the system through t he work done by his leg muscles. If you look at the big picture, you can say that the man is converting chemical potential energy in his body into mechanical energy of the bouncing system.

Hwa Chong Institution Sec 3 3 Extra: You should note that there is an optimum position for the leg to exert an upward force, and that position is at the lowest point of the trajectory, i.e. just when the man is about to bounce upward. This is not a coincidence; it is a well-studied phenomenon called resonance. We will be discussing more resonance when we learn of waves and oscillations. 8 When you jump from the ground into the air, where does your kinetic energy come from? What force does work on you to lift you into the air? The energy source is you. The chemical potential energy in your leg muscles is converted into elastic potential energy in a tensed position. When you released that tension, the foot exerts a normal contact force on the ground that is greater than the weight of your body. (During the push-off phase, the total force your feet exert on the ground is your weight + the extra push-off force). According to Newton's third law, the ground then exerts an equal but opposite force on you. Hence, during that push -off phase, the total f orce on you is a normal contact force from the ground minus your weight , and the resultant is directed upward. According to the work - energy principle, the work done by the resultant upward force is converted to the kinetic energy of your body. The upward force is the normal contact force from the ground on you. 9 Hydroelectric energy comes from gravity pulling down water through dams in rivers. Explain how such energy is just a form of solar energy by tracing how the Sun's energy can get the water from the ocean to the reservoir behind the dam. This question is asking you the basic principle behind the water cycle. First, the thermal energy of the Sun causes water to evaporate and form clouds in the atmosphere. Later, the water is precipitated in the rain on higher grounds , flowing down in rivers, and eventually stored behind the dam. Let's take a closer look at the energy cycle in the whole process. The energy delivered by the Sun is converted to the latent heat of vapori sation and work done against the atmosphere during the initial evaporation. The latent heat of vaporisation is the amount of energy required to convert a liquid into a gas . Work done against the atmosphere is the energy needed to expand the volume occupied when the water conver ts f rom liquid to gas at atmospheric pressure. When the water vapour rises to a height to form clouds, the latent heat of vapori sation is released back to the environment a s the water condenses in the form of droplets . The work done against the atmosphere is converted to gravitati onal potential energy because the water has risen to a reasonable height. When the water droplets fall in the rain, some GPE is lost beca