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 lesson.
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.
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 lesson 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 Lesson 4, and the ball can be treated as a particle. If, instead, you flip a baseball bat into the air (Fig. 9-1b), its motion is more complicated. Because every part of the bat moves differently, along paths of many different shapes, you cannot represent the bat as a particle. Instead, it is a system of particles each of which follows its own path through the air. However, the bat has one special point—the center of mass—that does move in a simple parabolic path. The other parts of the bat move around the center of mass. (To locate the center of mass, balance the bat on an outstretched finger; the point is above your finger, on the bat’s central axis.)
You cannot make a career of flipping baseball bats into the air, but you can make a career of advising long-jumpers or dancers on how to leap properly into the air while either moving their arms and legs or rotating their torso. Your starting point would be the person’s center of mass because of its simple motion.
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