Chapter 12

### INTRODUCTION

After completing this chapter, you will be able to:

• Identify Newton's laws of motion and gravitation and describe practical -illustrations of the laws.

• Explain what factors affect friction and discuss the role of friction in daily -activities and sports.

• Define impulse and momentum and explain the relationship between them.

• Explain what factors govern the outcome of a collision between two bodies.

• Discuss the relationships among mechanical work, power, and energy.

• Solve quantitative problems related to kinetic concepts.

What can people do to improve traction when walking on icy streets? Why do some balls bounce higher on one surface than on another? How can football linemen push larger opponents backward? In this chapter, we introduce the topic of kinetics with a discussion of some important basic concepts and principles relating to linear kinetics.

### NEWTON'S LAWS

Sir Isaac Newton (1642–1727) discovered many of the fundamental relationships that form the foundation for the field of modern mechanics. These principles highlight the interrelationships among the basic kinetic quantities introduced in Chapter 3.

#### Law of Inertia

Newton's first law of motion is known as the law of inertia. This law states the following:

A body will maintain a state of rest or constant velocity unless acted on by an external force that changes the state.

In other words, a motionless object will remain motionless unless there is a net force (a force not counteracted by another force) acting on it. Similarly, a body traveling with a constant speed along a straight path will continue its motion unless acted on by a net force that alters either the speed or the direction of the motion.

It seems intuitively obvious that an object in a static (motionless) situation will remain motionless barring the action of some external force. We assume that a piece of furniture such as a chair will maintain a fixed position unless pushed or pulled by a person exerting a net force to cause its motion. When a body is traveling with a constant velocity, however, the enactment of the law of inertia is not so obvious, because, in most situations, external forces do act to reduce velocity. For example, the law of inertia implies that a skater gliding on ice will continue gliding with the same speed and in the same direction, barring the action of an external force. But in reality, friction and air resistance are two forces normally present that act to slow skaters and other moving bodies.

A skater has a tendency to continue gliding with constant speed and direction because of inertia. Karl Weatherly/Getty Images.

#### Law of Acceleration

Newton's second law of motion is an expression of the interrelationships among force, mass, ...

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