p140w07_ct_16 - Racquetball Striking a Wall Copyright Loren...

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Physics 140: Winter 2007 Lecture #16 March 6, 2007 Dave Winn Racquetball Striking a Wall Copyright: Loren M. Winters Mt. Etna Andrew Davidhazy
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Exam information Second exam: Thursday March 8 th from 6-7:30 PM We will start promptly Bring a calculator Bring two 3”x5” cards with whatever you want to remember (both sides OK) Exam covers chapter 6-10.2. Note, Angular momentum and rolling motion will NOT be covered on this midterm! Where to take the exam? – Mark Skalsey or Arthur Cole : 1800 Chem – Fred Bechetti: 1400 Chem – Konstantin Bobkov: 170 Den – Dave Winn: 182 Den – Ctirad Uher: Angel Hall Auditorium C – Alternate Exam: 340 West Hall (same times and people as exam 1)
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Moment Sheet on Exam
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Torque and rolling motion θ mg F s F N ( ) () ατ θ I rF mg F F ma F mg F s N along s along = = Σ = = Σ = = Σ 0 cos sin 2 r Ia r I F s = = α () () ( ) 2 2 1 sin sin mr I g a r Ia ma mg along along + = + = If I = 1/2mr 2 then: ( ) () sin 1 sin 3 2 2 2 2 1 g mr mr g a along = + = Independent of mass!
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Imagine I roll two objects without slipping down a plane. One is a solid cylinder and one is a hollow cylinder. Both have the same radius. Which object reaches the bottom first? θ θ 1: The solid cylinder 2: The hollow cylinder 3: They tie ( ) ( ) () () θ sin sin 1 sin 2 1 3 2 2 g a g a mr I g a hollow solid along = = + = 2 2 1 Solid mr I = 2 Hollow mr I =
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Rolling and energy • As it rolls down, the object trades gravitational potential energy for kinetic energy • Friction does no work (acts over no distance), so all PE becomes KE •K E h a s t w o p a r t s : translational and rotational 2 4 3 2 4 1 2 2 1 2 2 2 2 2 1 2 2 1 2 2 1 r and mv mv mv KE v mr I I mv KE = + = = = + = ω Since PE is going into two forms, less goes into translation. The rolling object descends slower than a sliding object…
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We will race two identically shaped cans of Campbell’s soup down the slope. One is a very thin broth and the other is the nearly solid New England clam chowder. Which will win the race?
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p140w07_ct_16 - Racquetball Striking a Wall Copyright Loren...

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