Chapter07 - Chapter7:PotentialEnergy andEnergyConservation

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Chapter 7: Potential Energy  and Energy Conservation
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About Potential Energy Potential energy The energy stored within a physical system as a result of the position or configuration of the different parts of that system. It has the potential to be converted into other forms of energy, such as kinetic energy, and to do work in the process. The potential energy is a function of the state a system is in, defined relative to an arbitrary reference energy.
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1. Gravitational Potential Energy Work done by gravity Gravitational potential energy (near Earth’s surface) Then, W gravity = r F r s = ( - mg ö j ) (( y 2 - y 1 ) ö j ) = mgy 1 - mgy 2 U mgy = 1 2 2 1 ( ) gravity W U U U U U = - = - - = -∆
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Conservation of Mechanical Energy For gravitational force 2 1 2 1 ( ) gravity tot W W K K U U = = - = - - 1 1 2 2 K U K U + = + constant E K U = + = Conservation of Mechanical Energy 2 1 2 E K U mv mgy = + = +
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Since and , the potential energy is r ref is the position where the potential energy is zero For gravitational force at the Earth’s surface W U = -∆ W F dr = r r ref r r U F dr = - r r 0 ( ) y U mg dy mgy = - - =
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Problem 7.5) A baseball is thrown from the roof of 22.0-m tall building with an initial velocity of magnitude 12.0 m/s and directed at an angle of 53.1° above the horizontal. a) What is the speed of the ball just before it strikes the ground?
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Chapter07 - Chapter7:PotentialEnergy andEnergyConservation

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