5 - The Laws of Motion

# 5 The Laws of - Chapter 5 The Laws of Motion CHAPTE R OUTLI N E 5.1 The Concept of Force 5.2 Newtons First Law and Inertial Frames 5.3 Mass 5.4

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The Laws of Motion ± A small tugboat exerts a force on a large ship, causing it to move. How can such a small boat move such a large object? (Steve Raymer/CORBIS) Chapter 5 111 CHAPTER OUTLINE 5.1 The Concept of Force 5.2 Newton’s First Law and Inertial Frames 5.3 Mass 5.4 Newton’s Second Law 5.5 The Gravitational Force and Weight 5.6 Newton’s Third Law 5.7 Some Applications of Newton’s Laws 5.8 Forces of Friction

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112 I n Chapters 2 and 4, we described motion in terms of position, velocity, and accelera- tion without considering what might cause that motion. Now we consider the cause— what might cause one object to remain at rest and another object to accelerate? The two main factors we need to consider are the forces acting on an object and the mass of the object. We discuss the three basic laws of motion, which deal with forces and masses and were formulated more than three centuries ago by Isaac Newton. Once we understand these laws, we can answer such questions as “What mechanism changes motion?” and “Why do some objects accelerate more than others?” 5.1 The Concept of Force Everyone has a basic understanding of the concept of force from everyday experience. When you push your empty dinner plate away, you exert a force on it. Similarly, you ex- ert a force on a ball when you throw or kick it. In these examples, the word force is asso- ciated with muscular activity and some change in the velocity of an object. Forces do not always cause motion, however. For example, as you sit reading this book, a gravita- tional force acts on your body and yet you remain stationary. As a second example, you can push (in other words, exert a force) on a large boulder and not be able to move it. What force (if any) causes the Moon to orbit the Earth? Newton answered this and related questions by stating that forces are what cause any change in the velocity of an object. The Moon’s velocity is not constant because it moves in a nearly circular orbit around the Earth. We now know that this change in velocity is caused by the gravita- tional force exerted by the Earth on the Moon. Because only a force can cause a change in velocity, we can think of force as that which causes an object to accelerate. In this chapter, we are concerned with the relationship between the force exerted on an ob- ject and the acceleration of that object. What happens when several forces act simultaneously on an object? In this case, the object accelerates only if the net force acting on it is not equal to zero. The net force acting on an object is deﬁned as the vector sum of all forces acting on the object. (We sometimes refer to the net force as the total force , the resultant force , or the unbalanced force .) If the net force exerted on an object is zero, the acceleration of the object is zero and its velocity remains constant. That is, if the net force acting on the ob- ject is zero, the object either remains at rest or continues to move with constant veloc- ity. When the velocity of an object is constant (including when the object is at rest), the
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## This note was uploaded on 02/24/2011 for the course PHYS 102 taught by Professor Wang during the Spring '11 term at Nanjing University.

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5 The Laws of - Chapter 5 The Laws of Motion CHAPTE R OUTLI N E 5.1 The Concept of Force 5.2 Newtons First Law and Inertial Frames 5.3 Mass 5.4

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