Lecture 10 PowerPoint

# Lecture 10 PowerPoint - Today Newtons third law nished...

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1 Today Newton’s third law finished Interaction with 2nd law Intro to Energy O ffi ce hours today only: 11:00 -12:00 (starting in the foyer after the lecture, then Henn 415)

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3 W6 Q2 You are lifting two 6-kg buckets that are attached to each other by a lightweight rope. 1. What are the tensions T 1 and T 2 when you lift the buckets at constant speed? 2. At the start of the motion, the buckets have an upward constant acceleration of magnitude 3.0 m/s 2 . What are the tensions now? T 1 T 2

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Solution 1) Look at the free-body diagrams on the left. At constant speed, a = 0. There is just weight and tension (exerted by 1 on 2) acting on bucket 2 (the lower one). So: T 12 – m 2 g = 0, so T 12 = m 2 g = 59 N There are three forces acting on bucket 1, tension T 1 , the weight and T 21 (force of the second bucket on the first). Third law: T 21 = T 12 The net force is thus T 1 – m 1 g – T 21 = 0, so T 1 = m 1 g + T 21 = 118 N 2) Same free-body diagrams but now a = 3.0 m/s 2 .
• Spring '11
• ERIC

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