9781118230725.pdf

258 CHAPTER 10 ROTATION

What Is Physics?

As we have discussed, one focus of physics is motion. However, so far we

have examined only the motion of translation,in which an object moves along

a straight or curved line, as in Fig. 10-1a. We now turn to the motion of rotation,

in which an object turns about an axis, as in Fig. 10-1b.

You see rotation in nearly every machine, you use it every time you open a

beverage can with a pull tab, and you pay to experience it every time you go to an

amusement park. Rotation is the key to many fun activities, such as hitting a long

drive in golf (the ball needs to rotate in order for the air to keep it aloft longer)

and throwing a curveball in baseball (the ball needs to rotate in order for the air

to push it left or right). Rotation is also the key to more serious matters, such as

metal failure in aging airplanes.

We begin our discussion of rotation by defining the variables for the

motion, just as we did for translation in Chapter 2. As we shall see, the vari-

ables for rotation are analogous to those for one-dimensional motion and, as

in Chapter 2, an important special situation is where the acceleration (here the

rotational acceleration) is constant. We shall also see that Newton’s second

law can be written for rotational motion, but we must use a new quantity

called torque instead of just force. Work and the work–kinetic energy

theorem can also be applied to rotational motion, but we must use a new quan-

tity called rotational inertiainstead of just mass. In short, much of what we

have discussed so far can be applied to rotational motion with, perhaps, a few

changes.

Caution:In spite of this repetition of physics ideas, many students find this

and the next chapter very challenging. Instructors have a variety of reasons as

to why, but two reasons stand out: (1) There are a lot of symbols (with Greek

Figure 10-1Figure skater Sasha Cohen in motion of (a) pure translation in a fixed

direction and (b) pure rotation about a vertical axis.

(a) (b)

Mike Segar/Reuters/Landov LLC Elsa/Getty Images, Inc.