Chapter 12, Problem 56PQ

Disc jockeys (DJs) use a turntable in applying their trade, often using their hand to speed up or slow down a disc record so as to produce a desired change in the sound (Fig. P12.56). Suppose DJ Trick wants to slow down a record initially rotating clockwise (as viewed from above) with an angular speed of 33.0 rpm to an angular speed of 22.0 rpm. The record has a rotational inertia of 0.012 kg·m² and a radius of 0.15 m. a. What angular acceleration is necessary if he wishes to accomplish this feat in exactly 0.65 s with a constant acceleration? b. How many revolutions does the record go through during this change in speed? c. If DJ Trick applies a vertical force with his finger to the edge of the record, with what force must he push so as to slow the record in the above time? Assume the coefficient of kinetic friction between his finger and the record is 0.50, and ignore the mass of the finger.

FIGURE P12.56