p140w07_ct_12 - Racquetball Striking a Wall Copyright:...

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Physics 140: Winter 2007 Lecture #12 February 13, 2007 Dave Winn Racquetball Striking a Wall Copyright: Loren M. Winters Mt. Etna Andrew Davidhazy
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Last time: 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 • Potential energy: energy stored in interaction (gravitational, elastic) Δ PE = -W v KE= 1 2 mv 2 Energy concept is powerful, but can’t be a complete description of motion: it lacks any direction information
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Conservation of Energy • Energy is never created or destroyed • Work changes the form of energy • Non-conservative forces change energy into a form that cannot easily be recovered (eg. Heat) Gravity and spring force store energy temporarily Work done by friction creates heat, which cannot easily be converted back Why is energy conserved?
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Emmy Noether: 1882-1935 If the laws of nature exhibit a symmetry, there will be a conserved quantity associated with this symmetry. Are there any other symmetries in nature? The force of gravity is the same today as it was yesterday, and it will be the same again tomorrow This is a “time symmetry” This leads directly to conservation of energy
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Symmetry and Conservation • The laws of physics are the same here as they are over there (translational symmetry) • The laws of physics are the same no matter which direction I look (rotational symmetry) Conservation of Linear Momentum (today) Conservation of Angular Momentum (after midterm 2)
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• Momentum ( ) is a vector measure of motion • Its value is dependent of the direction of motion • For an object of mass m moving with velocity • Depends on both mass and velocity: motion and inertia • Does depend on direction of motion! v
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p140w07_ct_12 - Racquetball Striking a Wall Copyright:...

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