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Warmup Be prepared to answer the following: 1)What is required to turn an object travelling in a straight line? 2)What is required to rotate a stationary object? 3)What is the difference between a rotation and a revolution, and how are they related to different kinds of velocities?

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Rotational Motion DECEMBER 12 TH, 2011

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Rotational Motion Summary 3 v v Angular Velocity Linear Velocity v = r Centripetal Acceleration Centripetal Force

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Rotational Motion Summary Torque (F t = Fd) Center of mass and stability Torque is a twisting force that causes an object to rotate. Unlike Work, the Force and the Distance in Torque are in perpendicular directions. A center of mass is the average point of mass in an object. It’s possible to be located outside the object! Stability is determined by whether or not the center of mass is above the confines of the supporting base. Unstable Stable

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Rotational Motion Summary Linear MotionCircular Motion I = m I = mr 2 p = m*vp a = mr 2 ω Compare: Force causes linear acceleration. Torque, which is based on leveraged force, causes rotational acceleration, therefore, the radius of the mass as compared to the center of mass is a factor in rotational inertia and rotational momentum.

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Centripetal vs. Centrifugal 1 Car going around a curve. You are sitting on the seat next to the outside door. What is the direction of the resultant force on you as you turn? Is it away from center or toward center of the turn? Force ON you is toward the center. FcFcFcFc

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Centripetal vs. Centrifugal 1 (cont.) There is an outward force, but it does not act ON you. It is the reaction force exerted BY you ON the door. It affects only the door. The centripetal force is exerted BY the door ON you. (Centrally) FcFc F’Reaction

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Centripetal vs. Centrifugal 2 Disappearing platform at fair. R What exerts the centripetal force in this example and on what does it act? The centripetal force is exerted BY the wall ON the man. A reaction force is exerted by the man on the wall, but that does not determine the motion of the man. FcFcFcFc

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Centripetal vs. Centrifugal 3 How is the water removed from clothes during the spin cycle of a washer? Think carefully before answering... Does the centripetal force throw water off the clothes? NO. Actually, it is the LACK of a force that allows the water to leave the clothes through holes in the circular wall of the rotating washer.

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Study Sequence 10 1)Make flashcards for units and study with a partner until you’re confident you can match them at 100% accuracy. 2)Practice conversions using dimensional analysis. Make sure you know the most commonly-used prefixes in the metric system (kilo, centi, milli, deci, deca) 3)Practice simplification of algebraic expressions (Algebra, Chapter 2, try a few problems from each section, check answers in the back) 4)Practice momentum and energy problems (Physics, Chapters 9-10)

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