Physics 150 Fall 2003
Lect. 5. Galileo & Kepler to Newton - Universal Laws of Classical Mechanics
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Summary
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Definitions: displacement, velocity, acceleration, momentum are vectors that describe motion
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Time line - Renaissance - Copernicus, Galileo, Kepler, Newton
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Newton’s three laws:
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1. A body moves with constant velocity unless acted upon by a force
-- equivalent to principle of inertia
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2. F=ma
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Equal and opposite forces -- action/reaction
(equivalent to conservation of momentum - more later)
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Concept of Force, Mass
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Mass is the property of a body - never changes for that body
- a scalar measure of resistance to acceleration
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Force is a vector - tends to cause acceleration (change of momentum)
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The force in Newton’s equation is the "Net Force"
-- the vector sum of all forces on a body
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Demonstrations of Laws
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Curved and Circular motion
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Curved motion is accelerated motion
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Motion at constant speed in a circle: a = v2/R
toward the center (centripetal)
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Motion at constant speed in a circle can occur only if there
is a centripetal force: F = ma = mv2/R
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Universal Law of gravitation
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Force making planets move in circles (or elipses) is the force of gravity
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Together with Kepler's third law, one finds F decreases as 1/R2
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Universal law of gravitation: F = G M m /r2
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The falling Apple and the Moon: each accelerates toward the earth obeying the same law!
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Enormous impact upon Western Thought
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Examples of the huge effects of the tiny force of gravity
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Next Time: Conservation Laws -- Even
more important than Newton’s Equations!
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Read for Next Time: March, Chapt. 5 and Lightman Chapt. 1.