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

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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 Doesnt 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 ( ) Fds = s d F cos r r ( ) ds F = Fds along cos A basic example: F || to motion F s If no other forces act, what happens?...
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p140w07_ct_10 - Physics 140: Winter 2007 Lecture #10...

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