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# chapter_4 - Forces exerted by Earth wind and water properly...

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81 4 CHAPTER The Laws of Motion O U T L I N E 4.1 Forces 4.2 Newton’s First Law 4.3 Newton’s Second Law 4.4 Newton’s Third Law 4.5 Applications of Newton’s Laws 4.6 Forces of Friction © Royalty-Free/Corbis Classical mechanics describes the relationship between the motion of objects found in our everyday world and the forces acting on them. As long as the system under study doesn’t in- volve objects comparable in size to an atom or traveling close to the speed of light, classical mechanics provides an excellent description of nature. This chapter introduces Newton’s three laws of motion and his law of gravity. The three laws are simple and sensible. The first law states that a force must be applied to an object in order to change its velocity. Changing an object’s velocity means accelerating it, which implies a relationship between force and acceleration. This relationship, the second law, states that the net force on an object equals the object’s mass times its acceleration. Finally, the third law says that whenever we push on something, it pushes back with equal force in the opposite direction. These are the three laws in a nutshell. Newton’s three laws, together with his invention of calculus, opened avenues of inquiry and discovery that are used routinely today in virtually all areas of mathematics, science, engi- neering, and technology. Newton’s theory of universal gravitation had a similar impact, start- ing a revolution in celestial mechanics and astronomy that continues to this day. With the ad- vent of this theory, the orbits of all the planets could be calculated to high precision and the tides understood. The theory even led to the prediction of “dark stars,” now called black holes, over two centuries before any evidence for their existence was observed. 1 Newton’s three laws of motion, together with his law of gravitation, are considered among the greatest achievements of the human mind. 4.1 FORCES A force is commonly imagined as a push or a pull on some object, perhaps rapidly, as when we hit a tennis ball with a racket. (See Figure 4.1.) We can hit the ball at different speeds and direct it into different parts of the opponent’s court. This Forces exerted by Earth, wind, and water, properly channeled by the strength and skill of these windsurfers, combine to create a non-zero net force on their surfboards, driving them forward through the waves. 1 In 1783, John Michell combined Newton’s theory of light and theory of gravitation, predicting the existence of “dark stars” from which light itself couldn’t escape. Figure 4.1 Tennis champion Andy Roddick strikes the ball with his racket, applying a force and directing the ball into the open part of the court. © Lorenzo Ciniglio/Corbis

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82 Chapter 4 The Laws of Motion means that we can control the magnitude of the applied force and also its direc- tion, so force is a vector quantity, just like velocity and acceleration.
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