CIRCULAR MOTION Objectives • Describe the two types of CIRCULAR MOTION circular motion. (10.1) 10 • Describe the relationship among tangential speed, rotational speed, and radial THE BIG distance. (10.2) Centripetal force keeps an • Describe the factors that affect IDEA ........ object in circular motion. the centripetal force acting on an object. (10.3) • Explain the “centrifugal-force hich moves faster on a merry-go- effect.” (10.4) round, a horse near the outside • Explain why centrifugal force rail or one near the inside rail? is not considered a true force. W If you swing a tin can at the end of a (10.5) string in a circle over your head and the string breaks, does the can fly directly This chapter entirely omits the outward, or does it continue its motion “right hand rule,” where fingers without changing its direction? While a of the right hand represent the motion of a rotating body hamster rotates its cage about an axis, and the thumb represents the does the hamster rotate or does it revolve positive vector of motion. Please about the same axis? These questions indi- spare undue emphasis on this cate the flavor of this chapter. We begin by material, which your students can get into in a later course. discussing the difference between rotation and revolution. discover! MATERIALS marble, paper plate, scissors EXPECTED OUTCOME Students will observe that the marble moves in a circular path along the rim of the plate. ANALYZE AND CONCLUDE discover! 1. See Expected Outcome. The marble moves off in a Why Do Objects Move in Circles? Analyze and Conclude straight line where the rim 1. Roll a marble around the rim of a paper plate. 1. Observing Describe the motion of the marble is not present. 2. Using a pair of scissors, cut a 90º wedge- before and after a section of the plate was 2. Apply a series of inward shaped piece from the plate. removed. pushes on the marble. 3. Roll the marble around the rim of the modi- 2. Predicting What could you do to keep the 3. An inward directed force is fied plate. marble moving in a circle? necessary to keep an object 3. Making Generalizations What is required to moving in a circle. keep any object moving in a circle? 170 170 10.1 Rotation and 10.1 Rotation and Revolution Revolution Both the Ferris wheel shown in Figure 10.1 and an ice skater doing Key Terms axis, rotation, revolution a pirouette turn around an axis. An axis is the straight line around which rotation takes place. Teaching Tip Distinguish between a rotation (spin about FIGURE 10.1 an axis located within a body) The Ferris wheel turns and a revolution (movement about an axis. around an axis outside the body). A wheel rotates; its rim revolves. Describe the case of a satellite spinning (rotating) while it orbits Earth (revolving). Ask Does a tossed football rotate or revolve? It rotates. Does a ball whirled overhead at the end of a string rotate or revolve? It revolves about you. CONCEPT Two types of circular CHECK ...... motion are rotation and revolution. Teaching Resources Two types of circular motion are rotation and revolution. • PresentationEXPRESS When an object turns about an internal axis—that is, an axis located You rotate about an internal axis when • Interactive Textbook within the body of the object—the motion is called rotation, or you spin. You revolve spin. Both the Ferris wheel and the skater rotate. When an object • Conceptual Physics Alive! around an external axis DVDs Rotation turns about an external axis, the motion is called revolution. when you circle about Although the Ferris wheel rotates, the riders revolve about its axis. that axis. Earth undergoes both types of rotational motion. It revolves 1 10.1.1 10.2 Rotational around the sun once every 365 4 days, and it rotates around an axis passing through its geographical poles once every 24 hours.10.1.2 Speed CONCEPT Key Terms What are two types of circular motion? linear speed, tangential speed, CHECK ...... rotational speed Common Misconceptions 10.2 Rotational Speed Linear speed and rotational speed are the same. We began this chapter by asking which moves faster on a merry- go-round, a horse near the outside rail or one near the inside rail. FACT Linear speed is the distance moved per unit of time while Similarly, which part of a turntable moves faster? On the pre-CD rotational speed is the number of record player shown in Figure 10.2, which part of the record moves FIGURE 10.2 ᭡ rotations per unit of time. faster under the stylus—the outer part where the ladybug sits or a The turntable rotates around The linear speed on a rotating part near the orange center? If you ask people these questions you’ll its axis while a ladybug sit- surface is the same at all radial probably get more than one answer, because some people will think ting at its edge revolves distances. around the same axis. about linear speed while others will think about rotational speed. FACT Linear speed varies with the distance from the axis of rotation. CHAPTER 10 CIRCULAR MOTION 171 171 Teaching Tip Distinguish between rotational speed and linear speed by describing the think! Types of Speed Linear speed, which we simply called speed in examples of riding at different Chapter 4, is the distance traveled per unit of time. A point on the radial positions on a merry-go- At an amusement park, outer edge of a merry-go-round or turntable travels a greater dis- round. When the linear speed you and a friend sit on a large rotating disk. You sit tance in one complete rotation than a point near the center. So the is tangent to a curved path, we linear speed is greater on the outer edge of a rotating object than it speak of tangential speed. at the edge and have a rotational speed of 4 RPM is closer to the axis. The speed of something moving along a circular and a linear speed of path can be called tangential speed because the direction of motion Demonstration 6 m/s. Your friend sits is always tangent to the circle. For circular motion, we can use the halfway to the center. terms linear speed and tangential speed interchangeably. Place two coins on the top of a What is her rotational turntable, one near the center speed? What is her linear Rotational speed (sometimes called angular speed) is the num- and the other near the edge. speed? ber of rotations per unit of time. All parts of the rigid merry-go- Rotate the turntable and Answer: 10.2.1 round and the turntable rotate about their axis in the same amount show that the outer coin has of time. Thus, all parts have the same rate of rotation, or the same a greater linear speed. Explain that both coins have the number of rotations per unit of time. It is common to express rota- same rotational speed—they tional speed in revolutions per minute (RPM).10.2.1 For example, pho- undergo the same number of 1 nograph turntables that were common in the past rotate at 333 RPM. revolutions per second. (Or, A ladybug sitting anywhere on the surface of the turntable in Figure stick two pieces of clay to the 10.3 revolves at 33 1 RPM. turntable, one near the center 3 and one near the edge. Place FIGURE 10.3 ᭤ pencils upright in the clay and All parts of the turntable let the turntable rotate. The rotate at the same rota- different tangential speeds tional speed. a. A point will be evident.) farther away from the center travels a longer path in the same time Teaching Tip Compare and therefore has a the speeds of the coins to the greater tangential speed. speeds of different parts of an b. A ladybug sitting old phonograph record beneath twice as far from the cen- ab the stylus. Linear velocity ter moves twice as fast. that is perpendicular to the radial direction is the same as tangential velocity (v 5 rv). Give examples such as being able to Tangential and Rotational Speed Tangential speed and rota- see detail on the hub cap of a tional speed are related. Have you ever ridden on a giant rotating moving car while not seeing platform in an amusement park? The faster it turns, the faster your similar detail on the tire. tangential speed is. Tangential speed is directly proportional to the Ask If a meter stick rotational speed and the radial distance from the axis of rotation. So supported at the 0-cm mark 10.2.2 swings like a pendulum from we state your fingers, how fast at any Tangential speed radial distance rotational speed given moment is the 100-cm We use the symbol mark moving compared to the In symbol form, ~ to mean directly 50-cm mark? The 100-cm mark v r/ is twice as far from the center of proportional. rotation as the 50-cm mark and where v is tangential speed and / (pronounced oh MAY guh) is rota- thus has twice the linear speed. tional speed. You move faster if the rate of rotation increases (bigger /). You also move faster if you are farther from the axis (bigger r). 172 172 discover! discover! MATERIALS cylindrical can How Does Linear Speed Depend EXPECTED OUTCOME The on Radius? tapered cup will follow a curved path as it rolls. 1. Roll a cylindrical can across a table.