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# lecture17 - Statics(II More examples with friction Internal...

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1 Physics 1D03 - Lecture 17 Statics (II) Serway 12.3 More examples with friction Internal forces in simple structures Practice problems: Chapter 12, Problems 47, 51, 52. Physics 1D03 - Lecture 17 If there is no friction at the floor, a ladder cannot lean against a vertical wall in equilibrium, even with friction at the wall. Does the problem change if the ladder leans against the top of the wall? What does the free- body diagram look like? θ A B μ A = 0 μ B w

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2 Physics 1D03 - Lecture 17 Example: rolling a spool. inner radius a, outer radius b = 2 a μ s = 0.60 at both surfaces If the spool has weight w = 100 N, what minimum force P is needed to rotate the spool? b a C B A P Plan: Assume the spool is not rotating ( still in equilibrium ) but is about to slip (so f s = μ s N at each contact). Use the conditions for equilibrium, and the two friction equations, to solve for P . Physics 1D03 - Lecture 17 1) Free-body diagram: b a C B A b = 2 a w = 100 N μ s = 0.60 y x 2) Equilibrium equations: F x : F y : torques about C: 3) about to slip:
3 Physics 1D03 - Lecture 17 b

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