7 - Energy and Energy Transfer

7 - Energy and Energy Transfer - Chapter 7 Energy and...

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Chapter 7 Energy and Energy Transfer ± On a wind farm, the moving air does work on the blades of the windmills, causing the blades and the rotor of an electrical generator to rotate. Energy is transferred out of the sys- tem of the windmill by means of electricity. (Billy Hustace/Getty Images) CHAPTER OUTLINE 7. 1 Systems and Environments 7. 2 Work Done by a Constant Force 7. 3 The Scalar Product of Two Vectors 7. 4 Work Done by a Varying Force 7. 5 Kinetic Energy and the Work–Kinetic Energy Theorem 7. 6 The Nonisolated System— Conservation of Energy 7. 7 Situations Involving Kinetic Friction 7. 8 Power 7. 9 Energy and the Automobile 181
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T he concept of energy is one of the most important topics in science and engineer- ing. In everyday life, we think of energy in terms of fuel for transportation and heating, electricity for lights and appliances, and foods for consumption. However, these ideas do not really define energy. They merely tell us that fuels are needed to do a job and that those fuels provide us with something we call energy. The definitions of quantities such as position, velocity, acceleration, and force and associated principles such as Newton’s second law have allowed us to solve a variety of problems. Some problems that could theoretically be solved with Newton’s laws, how- ever, are very difficult in practice. These problems can be made much simpler with a different approach. In this and the following chapters, we will investigate this new ap- proach, which will include definitions of quantities that may not be familiar to you. Other quantities may sound familiar, but they may have more specific meanings in physics than in everyday life. We begin this discussion by exploring the notion of energy. Energy is present in the Universe in various forms. Every physical process that oc- curs in the Universe involves energy and energy transfers or transformations. Unfortu- nately, despite its extreme importance, energy cannot be easily defined. The variables in previous chapters were relatively concrete; we have everyday experience with veloci- ties and forces, for example. The notion of energy is more abstract, although we do have experiences with energy, such as running out of gasoline, or losing our electrical service if we forget to pay the utility bill. The concept of energy can be applied to the dynamics of a mechanical system without resorting to Newton’s laws. This “energy approach” to describing motion is especially useful when the force acting on a particle is not constant; in such a case, the acceleration is not constant, and we cannot apply the constant acceleration equations that were developed in Chapter 2. Particles in nature are often subject to forces that vary with the particles’ positions. These forces include gravita- tional forces and the force exerted on an object attached to a spring. We shall describe techniques for treating such situations with the help of an important con- cept called conservation of energy . This approach extends well beyond physics, and
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7 - Energy and Energy Transfer - Chapter 7 Energy and...

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