# Chapter 5 - Exercises and Problems - Page 90: 18

a) $m = 7.065 kg$ b) $m = 3.947 kg$

#### Work Step by Step

a) We assume that the pulley is massless. We know that the left mass is 2.1 kg. Thus, we treat this as a system, and we use Newton's second law to find: $-.64(m+2.1)=2.1gsin60-mgsin20\\ m = 7.065 kg$ b) We use a similar process to find: $.76(m+2.1)=2.1gsin60-mgsin20\\ m = 3.947 kg$

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