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Fundamentals of Physics by David Halliday, Robert Resnick, Jearl Walker

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The most dangerous type of collision between two cars is a head-on collision, which can be fatal to either or both drivers. Surprisingly, the data collected on head-on collisions suggest that the risk of fatality to a driver is less if that driver has a passenger in the car.

How can that be true if, in fact, it is true?

The answer is in this chapter.

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Fig. 9-1 (a) A ball tossed into the air follows a parabolic path. (b) The center of mass (black dot) of a baseball bat flipped into the air follows a parabolic path, but all other points of the bat follow more complicated curved paths.

9-1  What Is Physics?

Every mechanical engineer hired as an expert witness to reconstruct a traffic accident uses physics. Every trainer who coaches a ballerina on how to leap uses physics. Indeed, analyzing complicated motion of any sort requires simplification via an understanding of physics. In this chapter we discuss how the complicated motion of a system of objects, such as a car or a ballerina, can be simplified if we determine a special point of the system—the center of mass of that system.

Here is a quick example. If you toss a ball into the air without much spin on the ball (Fig. 9-1a), its motion is simple—it follows a parabolic path, as we discussed in Chapter 4, and the ball can be treated ...

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