Lect21

# Lect21 - PHYS 172 Modern Mechanics Fall 2009 Lecture 21...

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Fall 2009 PHYS 172: Modern Mechanics Lecture 21 – Nonzero Torque Systems Read 10.8–10.10

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Exam #3 is next week Tuesday November 17th at 8 pm in Elliott Hall Alternative testing at same time in Room 8 in Physics Lectures both days next week, and labs & recitations as usual
CLICKER QUESTION #1 Reading Question (Sections 10.8 – 10.10) (This is a closed-book quiz, no consulting with neighbors, etc.) Consider the gyroscope shown in the figure. Its axis is horizontal, and it is sweeping out a circle in the horizontal plane. Ignore the role of friction in slowing it down. Choose the most correct statement: A. The angular momentum of the gyroscope is constant. B. The net force on the gyroscope is zero. C. The net torque on the gyroscope is zero. D. (none of the above).

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Consider the gyroscope shown in the figure. Its axis is horizontal, and it is sweeping out a circle in the horizontal plane. Ignore the role of friction in slowing it down. Choose the most correct statement: A. The angular momentum of the gyroscope is constant. Remember, this is a vector. About the axis? The pivot point? B. The net force on the gyroscope is zero. Does the center of mass accelerate? Is the center of mass moving in circular motion? Is there a force in this direction? Does there have to be one? C. The net torque on the gyroscope is zero. Consider torque about pivot. Gravity? D. (none of the above).
Last Time A A L r p = ! r r r single particle system: multiparticle system: , A trans A rot L L L = + r r r , , trans A cm A tot L r P = ! r r r 1, 1 2, 2 rot cm cm L r p r p = ! + ! + r r r r r L 1 p r 2 p r 3 p r 1, cm r r 2, cm r r 3, cm r r , cm A r r ! p A r A s in A r " =

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Last Time Angular velocity ω : Moment of inertia I : 2 2 2 1 1 2 2 3 3 I mr m r m r ! ! ! = + + + L rot L I ! = r r
, net ext dP F dt = r r , , A net ext A dL dt ! = r r E W Q ! = + The three fundamental principles: What else we will use for describing rotations: A A net r F !" # r

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Lect21 - PHYS 172 Modern Mechanics Fall 2009 Lecture 21...

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