79. The initial height of the 2Mblock, shown in Fig. 8-64, is the y= 0 level in our computations of its value of Ug. As that block drops, the spring stretches accordingly. Also, the kinetic energy Ksysis evaluated for the system-- that is, for a total moving mass of 3M.(a) The conservation of energy, Eq. 8-17, leads to Ki+ Ui= Ksys+ Usys ¡0 + 0 = Ksys+ (2M)g(–0.090) + 12k(0.090)2.Thus, with M= 2.0 kg, we obtain
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