chapter_5

# chapter_5 - An asteroid plunges through Earth's atmosphere...

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118 5 CHAPTER Energy OUTLINE 5.1 Work 5.2 Kinetic Energy and the Work–Energy Theorem 5.3 Gravitational Potential Energy 5.4 Spring Potential Energy 5.5 Systems and Energy Conservation 5.6 Power 5.7 Work Done by a Varying Force NASA Energy is one of the most important concepts in the world of science. In everyday use, energy is associated with the fuel needed for transportation and heating, with electricity for lights and appliances, and with the foods we consume. These associations, however, don’t tell us what energy is , only what it does , and that producing it requires fuel. Our goal in this chapter, therefore, is to develop a better understanding of energy and how to quantify it. Energy is present in the Universe in a variety of forms, including mechanical, chemical, electromagnetic, and nuclear energy. Even the inert mass of everyday matter contains a very large amount of energy. Although energy can be transformed from one kind to another, all observations and experiments to date suggest that the total amount of energy in the Universe never changes. This is also true for an isolated system—a collection of objects that can exchange energy with each other, but not with the rest of the Universe. If one form of energy in an isolated system decreases, then another form of energy in the system must increase. For example, if the system consists of a motor connected to a battery, the battery converts chemical energy to electrical energy, and the motor converts electrical energy to mechanical energy. Understanding how energy changes from one form to another is essential in all the sciences. In this chapter the focus is mainly on mechanical energy , which is the sum of kinetic energy —the energy associated with motion—and potential energy —the energy associated with position. Using an energy approach to solve certain problems is often much easier than using forces and Newton’s three laws. These two very different approaches are linked through the concept of work . 5.1 WORK Work has a different meaning in physics than it does in everyday usage. In the physics deFnition, a programmer does very little work typing away at a computer. A mason, by contrast, may do a lot of work laying concrete blocks. In physics, work is An asteroid plunges through Earth's atmosphere while pterodactyls watch. This artist's conception is of a catastrophic event thought to have led to the extinction of dinosaurs. During an impact, an asteroid only a kilometer across releases its awesome energy of motion as heat and light, delivering the explosive equivalent of one hundred million atomic bombs.

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5.1 Work 119 done only if an object is moved through some displacement while a force is ap- plied to it. If either the force or displacement is doubled, the work is doubled.
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chapter_5 - An asteroid plunges through Earth's atmosphere...

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