p140w07_ct_05 - Racquetball Striking a Wall Copyright:...

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Physics 140: Winter 2007 Lecture #5 January 18, 2007 Dave Winn Racquetball Striking a Wall Copyright: Loren M. Winters Mt. Etna Andrew Davidhazy
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Dynamics: what do we need to explain? • How and why do things change? • First great realization: what constitutes change? • Old and incorrect idea : anything not sitting still needs to be explained, motion has to have a cause • Not a surprising notion: To keep something moving you need to keep pushing: motion implies a mover
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Galileo reveals another illusion… • In free fall, Galileo saw that air friction was a complication: worked to remove it • To understand other motions, friction must be removed as well • What does motion without friction look like?
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Newton’s first law Any body continues in a state of rest, or of uniform motion in a straight line, unless it is compelled to change its motion by forces imposed on it. • Newton’s first law defines inertial reference frames. • In an inertial frame objects move at constant velocity unless a force acts on them. • In a non-inertial (accelerating frame) objects appear to change velocity in the absence of applied force. Also called the “law of inertia”: tendency to remain the same FORCES change motion
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A car travels around a curve at a constant speed. Is there a net force acting on it? 1. No, its speed is constant 2. Yes 3. It depends on the speed of the car and the sharpness of the curve Any time the velocity changes there must be unbalanced forces acting
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The lessons of the first law • You don’t need to explain motion – Nothing is needed to keep a moving object moving • You need to explain changes in motion – When objects speed up, slow down, or change direction, there must be a cause • Unbalanced forces cause changes in motion – If motion is not changing, forces must balance – If motion is changing, there must be a net force
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1 st Law consequences in a car crash • Bumper hits wall, forces acting on it cause it to stop • You keep going until
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p140w07_ct_05 - Racquetball Striking a Wall Copyright:...

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