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Unformatted text preview: “HI-WM why.” .. 50 Chaptertl Force and Motion "V . . . it always rises to its [fit ball is released here . . . . . . so if the surface is made horizontal, the ball should roll forever: ElfillBEAJ Galileo considered bails rolling on inclines and concluded that a ball on a horizontal surface should roll forever. Here there’s a nonzero net force acting on the car, so the car’s motion is chaugin The three forces sum to zero, so the plane moves in a straight line with constant speed. ,F F 2 a at: net (b) flfiltREAJ The net force determines the change in an object's motion. starting height . . . And the answer seemed obvious: It took a force—a push or a pull—to keep something mov- ing. This idea makes sense: Stop exerting yourself when jogging, and you stop moving; take your foot off the gas pedal, and your car soon stops. Everyday experience seems to suggest that Aristotle was right, and most of us carry in our heads the Aristotelian idea that motion re- quires a cause—something that pushes or pulls on a moving object to keep it going. Actually, What keeps things moving? is the wrong question. In the early 1600s, Galileo Galilei did experiments that convinced him that a moving object has an intrinsic “quantity of motion” and needs no push to keep it moving (Fig. 4.1). Instead of answering What keeps things moving?, Galileo declared that the question needs no answer. In so doing, he set the stage for centuries of progress in physics, beginning with the achievements of Issac Newton and culminating in the work of Albert Einstein. The Right Question Our first question—Why does the spacecraft keep moving?—is the wrong question. So what’s the right question? It’s the second one, about why the baseball’s motion changed. The study of dynamics isn’t about what causes motion itself; it’s about what causes changes in motion. Changes include starting and stopping, speeding up and slowing down, and changing direction. Any change in motion begs an explanation, but motion itself does not. Get used to this important idea and you’ll have a much easier time with physics. But if you remain a “closet Aristotelian,” secretly looking for causes of motion, you’ll find it dif- ficult to understand and apply the simple laws that actually govern motion. Galileo identified the right question about motion. But it was Isaac Newton who formu— lated the quantitative laws describing how motion changes. We use those laws today for everything from designing antilock braking systems, to building skyscrapers, to guiding spacecraft. 4.2 Newton's First and Second Laws What caused the baseball’s motion to change? Obviously, it was the bat pushing the ball. We use the term force to describe a push or a pull. And the essence of dynamics is sim— ply this: We‘ll soon quantify this notion, writing equations and solving numerical problems. But the essential point is in the simple sentence above. If you want to change an object’s motion, you need to apply a force. If you see an object’s motion change, you know there’s a force acting. Contrary to Aristotle, and probably to your own intuitive sense as well, it does not take a force to keep something in unchanging motion; force is needed only to change an object’s motion. The Net Force You can push a ball left or right, up or down. Your car’s tires can push the ca! forward or backward, or make it round a curve. Force has direction and is a vector quantity. Further- more, more than one force can act on a single object. We call the individual forces on an object interaction forces because they always involve other objects interacting with the object in question. In Fig. 4.2a, for example, the interaction forces are exerted by the peo— ple pushing the car. In Fig. 4.2b the interaction forces include the force of air on the plane, the engine force from the hot exhaust gases, and Earth’s gravitational force. We now explore in more detail the relation between force and change in motion. Exper- iment shows that what matters is the net force, meaning the vector sum of all individual in teraction forces acting on an object. If the net force on an object is not zero, then the object’s motion must be changing—in direction or speed or both (Fig. 4.2a). If the net force on an object is zeroino matter what individual interaction forces contribute to the net force—then the object’s motion is unchanging (Fig. 4.2b). ...
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