18. (a) The derivation of the acceleration is found in § 11-4; Eq. 11-13 gives agIMRcomcom=−+102where the positive direction is upward. We use 2com/2R=where the radius is R= 0.32 m and M= 116 kg is the totalmass (thus including the fact that there are two disks) and obtain ()222001(/2)1//2ggaMRMRRR=++which yields a= –g/51 upon plugging in R0= R/10 = 0.032 m. Thus, the magnitude of the center of mass acceleration is 0.19 m/s2.(b) As observed in §11-4, our result in part (a) applies to both the descending and the rising yoyo motions. (c) The external forces on the center of mass consist of the cord tension (upward) and the
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