p140w07_ct_14

# p140w07_ct_14 - Racquetball Striking a Wall Copyright:...

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Physics 140: Winter 2007 Lecture #14 February 20, 2007 Dave Winn Racquetball Striking a Wall Copyright: Loren M. Winters Mt. Etna Andrew Davidhazy

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Introduction to rotational motion • We have assumed that it doesn’t matter where forces are applied to an object • We have treated everything like point objects: only move in translation • Real objects both translate and rotate • Real motions are combinations of translation and rotation: superposition • Like superposition of horizontal and vertical motion of projectiles… F push F f F N
Rotational Kinematics • To describe rotation we need angular position, velocity, and acceleration • Define position as the angle between a reference line on the object and one fixed in space • Rigid bodies about a fixed axis of rotation θ θ = angular position Δθ = angular displacement ω = d θ /dt = angular velocity α = d ω /dt = d 2 θ /dt 2 = angular acceleration

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Positions in rotational motion • Rotations must be measured in radian measure •2 π radians = 360 ° • 1 radian = 57.3 ° • Are these Δθ , ω , and α vectors? • As long as the axis is fixed, they can be positive or negative only… r s θ = s/r + - Angular velocity ω always in the direction of angular displacement Δθ Usually: CCW = + Usually: CW = -
This disk spins in the direction shown. It is slowing down. Using the convention we just described, which combination correctly expresses the signs of the angular velocity ω and angular acceleration α ? 1: ω = +, α = + 2: ω = -, α = + 3: ω = +, α = - 4: ω = -, α = -

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completes a trip around the circle once every 4s. If the mass of the ball is 2kg, what is the angular velocity of the ball? 1: 4
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## This note was uploaded on 04/04/2008 for the course PHYSICS 140 taught by Professor Evrard during the Fall '07 term at University of Michigan.

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p140w07_ct_14 - Racquetball Striking a Wall Copyright:...

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