involved in every type of sport and activity. For example, when a pitcher throws a ball, she exerts a force on the ball that causes the ball to move forward ( Figure 1 ). If the batter hits the ball, then the bat exerts a force on the ball to change its motion. An understanding of forces is essential for a scientiic description of our environment. In simple terms, a force is a push or a pull. Forces can cause objects to change their motion. When you push on a chair to tuck it under a desk, you change the motion of the chair. he direction of a force is very important. If you push straight down on a book on a desk, the efect is usually very diferent than if you push sideways or pull up. his means that force has direction, making it a vector quantity. In Unit 1, you studied a branch of mechanics called kinematics. Remember that kinematics is the study of how objects move without being concerned with why they move. In this unit, you will study dynamics. Dynamics explains why objects move the way they do. One way to understand why an object moves is to study the forces acting on it. hese forces can cause the object to start moving, speed up, slow down, or remain stationary. In this chapter, you will be introduced to diferent types of forces and the laws that govern them. Measuring Forces Isaac Newton discovered many of the concepts in this chapter. For this reason the unit of force, the newton, is named ater him. he newton (N) is a derived SI unit equal to 1 kg·m/s 2 . To measure force in the laboratory, you can use either a spring scale or a force sensor. A spring scale has a spring that stretches more when greater forces pull on it. A needle is attached to the spring to indicate the amount of force. Usually a spring scale must be zeroed (the reading must be set to zero when not pulling) before use. Most spring scales can only measure a pulling force. A force sensor is an electronic device that can be attached to a computer or used independently. his device pro- vides an accurate digital reading of a force and can even graph how the force changes over time. A force sensor can measure both pushes and pulls. Force Diagrams To understand why an object will remain at rest, start moving, or change its motion, you need to be able to draw diagrams that show clearly which forces are acting on the object. hese diagrams are essential, especially when several forces are acting on the object simultaneously. he irst type of force diagram is called a system diagram. A system diagram is a simple sketch of all the objects involved in a situation. For example, if you are liting a book up in the air, the system diagram will show your hand pulling up on the book ( Figure 2 ). Figure 1 Forces are all around you. newton (N) the SI unit of force (1 N = 1 kg·m/s 2 ) system diagram a simple sketch of all objects involved in a situation F a � F g � Figure 2 A system diagram is a sketch showing all the objects involved in a situation. A system diagram helps you determine which objects push or pull on other objects. Figure 3 The FBD for the book shown in Figure 2.
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