10-01-PotentialEnergy

10-01-PotentialEnergy - Chapter 8 Potential Energy and...

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1 Work done by gravity 0 90 cos o d Weight (a) If the object moves horizontally , d Weight o 90 cos Fd W d Weight Weight Weight o 180 d (b) If the object moves upward , 1 180 cos o d Weight Weight Weight o 0 d 1 0 cos o (c)If the object moves downward , gravity does NO work. gravity does Negative work. gravity does positive work. Chapter 8 Potential Energy and Conservation of Energy mgd Work mgd Work Work done by the gravitational force mgd W B gravityA The work depends only on the difference between the initial height h A and the final height h B . It does not depend on the path from A to B ! d = h B -h A When an object of mass m moves from point A to point B near the surface of the Earth, the work done by the gravity is ) ( ) ( B A A B h h mg h h mg If the object follows the zigzag path from point A to point B, mgd W B gravityA d = h B -h A no no no no no + ) ( ) ( B A A B h h mg h h mg the work done by the gravity is still
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2 The work done by gravity on a closed path is zero On a closed path A B C D A, mgh mgh W A D C B gravityA 0 ) ( 0 0 The concept of “Conservative Force” 1. A force is conservative when the work it does on an object moving from A to B is independent of the path between A and B. Or 2. A force is conservative when the work it does on an object moving along any closed path is zero. Conservative forces Gravity Spring forces Non-Conservative forces Friction Muscular forces A B path1 path2 W 1 =W 2 A W=0
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3 The work done by friction on a closed path is different
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10-01-PotentialEnergy - Chapter 8 Potential Energy and...

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