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293 W hen an extended object such as a wheel rotates about its axis, the motion cannot be analyzed by treating the object as a particle because at any given time different parts In dealing with a rotating object, analysis is greatly simplified by assuming that the object is rigid. A rigid object is one that is nondeformable—that is, the relative loca- tions of all particles of which the object is composed remain constant. All real objects 10.1 Angular Position, Velocity, and Acceleration Figure 10.1 illustrates an overhead view of a rotating compact disc. The disc is rotating r remains constant. As the particle moves along the circle from the reference line (! " 0), it moves through an arc of (10.1a) (10.1b) Note the dimensions of ! in Equation 10.1b. Because ! is the ratio of an arc length and the radius of the circle, it is a pure number. However, we commonly give ! the arti- radian (rad), where Because the circumference of a circle is 2#r, it follows from Equation 10.1b that 360° r/r) rad " 2# rad. (Also note that 2# rad corresponds one radian is the angle subtended by an arc length equal to the radius of the arc. ! " s r s " r
! Rigid object Reference line (a) O P r (b) O P Reference line r s u Figure 10.1 A compact disc rotating about a fixed axis through O perpendicular to the plane of the figure. (a) In order to define angular position for the disc, a fixed reference line is chosen. A particle at P is located at a distance r from the rotation axis at O. (b) As the disc rotates, point P moves through an arc length s on a circular path of radius r. |