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Lect21 - PHYS 172 Modern Mechanics Lecture 21 Angular...

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Fall 2011 PHYS 172: Modern Mechanics Lecture 21 – Angular Momentum Quantization Read 11.8 – 11.11 1
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Announcements Exam 3 will be held on Wednesday, Nov. 16 , 8-9 pm in Elliott Hall. Material through chapters 8, 9, 10, and 11 will be covered. There WILL be a lecture on Nov. 15 and 17. No lectures, labs, recitations, or homework the week of Thanksgiving . A practice exam 3 has been put in the Exams folder. 2
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Predicting Position with Rotation F=mg A light string is wrapped around disk of radius R and moment of inertia I that can freely spin around its fixed axis. The string is pulled with force F during time Δ t . Assume that the disk was initially at rest ( ω i =0) 1) What will be the angular speed ω f ? θ R I d ! L tot dt = ! ! net ! ! L tot = ! " net ! t Solution: I ! ! f " I ! ! i = I ! ! f = ! R # ! F $% t I ! f = RF " t ! f = RF " t I m 3
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Predicting Position with Rotation A light string is wrapped around disk of radius R and moment of inertia I that can freely spin around its fixed axis. The string is pulled with force F during time Δ t . Assume that the disk was initially at rest ( ω i =0) 1) What will be the angular speed ω f ? 2) How far ( Δ x ) will the end of string move? θ R I d ! L tot dt = ! ! net ! f = RF " t I See also examples in Section 11.8 Solution: ! aver " # $ # t ! " = # aver ! t ω changes linearly with time: !
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