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Lecture15

Lecture15 - PHYSICS 220 Lecture 15 Rotation and Equilibrium...

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Lecture 15 Purdue University, Physics 220 1 Lecture 15 Rotation and Equilibrium Textbook Sections 8.4 - 8.7 PHYSICS 220

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Lecture 15 Purdue University, Physics 220 2 Overview Last Lecture Rotational Kinetic Energy K rot = ½ I ω 2 Moment of Inertia I = Σ m i r i 2 Depends on Mass and axis of rotation Energy is conserved but need to include rotational energy too K rot = ½ I ω 2 Torque = Force that causes rotation τ = F r sin θ Work done by torque W = τ θ – Equilibrium Σ F = 0 Σ τ = 0 Today – Equilibrium Rolling Objects
Lecture 15 Purdue University, Physics 220 3 Questions A rod is lying on a table and has two equal but opposite forces acting on it. What is the net force on the rod? A) Up B) Down C) Zero Will the rod move? A) Yes B) No F F y x y-direction: Σ F y = ma y +F – F = 0 Yes, it rotates!

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Lecture 15 Purdue University, Physics 220 4 The picture below shows two people lifting a heavy log. Which of the two people is supporting the greatest weight? A) The person on the left is supporting the greatest weight B) The person on the right is supporting the greatest weight C) They are supporting the same weight Look at torque about center: F R L – F L L/2 = 0 F R = ½ F L Quiz mg F L F R L/2 L
Lecture 15 Purdue University, Physics 220 5 Static Equilibrium 1) Draw FBD 2) Choose Axis of rotation 3) Σ τ = 0 Rotational EQ F 1 (1.2) – mg (4.6) = 0 F 1 = 4.6 (50 *9.8) / 1.2 F 1 = 1880 N 4) Σ F = 0 Translational EQ F 1 – F 2 – mg = 0 F 2 = F 1 – mg = 1390 N mg F 1 F 2 A 50 kg diver stands at the end of a 4.6 m diving board. Neglecting the weight of the board, what is the force on the pivot 1.2 meters from the end?

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Lecture 15 Purdue University, Physics 220 6 Exercise
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