p140w07_ct_10

p140w07_ct_10 - Physics 140 Winter 2007 Lecture#10 February...

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Unformatted text preview: Physics 140: Winter 2007 Lecture #10 February 6, 2007 Dave Winn Racquetball Striking a Wall Copyright: Loren M. Winters Mt. Etna Andrew Davidhazy Exam #1 Results • Median score 30/40 (75%) • Absolute Grade Scale was created assuming an average of 70% • Thus, 28/40 is the C+/B- boundary Forces and accelerations • O n l y changes in motion matter, not just motion • Acceleration is one measure of changes in motion • Accelerations are caused by forces • Allows explanation of many things: – Motion of projectiles – Effects of friction and weight – The need for forces in circular motion – Motions of planets etc a m = F r r More to the description of motion • Huygens: a way to measure the “quantity of motion”, independent of direction • Vis viva: motion of life • Since described as “kinetic energy”: one of many forms of energy something may have • An object can blast apart, creating motion where the was none, with no net force! Kinetic Energy • K E i s a scalar measure of motion (how much motion is there) • Its value is independent of the direction of motion • For an object of mass m moving with velocity • Depends on mass • Depends on speed • Doesn’t depend on direction of motion 2 mv = v 2 1 KE r How does energy change? • Thomas Young: when a force acts along the direction of motion , it “does work” and changes the energy of an object • The amount of work by a force in a little motion ds is given by: • Imagine pushing an object along a frictionless table • Forces which act along motion will accelerate it, and change energy • F o r c e s ⊥ do not mg F N F push ( ) θ Fds = s d F cos r r ⋅ ( ) ds F = θ Fds along cos A basic example: F || to motion F Δ s If no other forces act, what happens?...
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p140w07_ct_10 - Physics 140 Winter 2007 Lecture#10 February...

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