PHY 121 Ch 7 Lecture

PHY 121 Ch 7 Lecture - Lecture 12 Tuesday, February 26 1...

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1 Lecture 12 Tuesday, February 26
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2 For the gravitational potential energy of a particle of mass m to be equal to mgy A. The y -axis zero must be higher than the position of the particle at any time. B. The y -axis zero must be placed on the ground. C. It doesn’t matter where the zero of the y -axis is located. D. The only force acting on the particle must be its weight.
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3 If both weight and the spring force act on an object… A. its potential energy is the sum of the gravitational and elastic potential energies. B. its potential energy is the product of the gravitational and elastic potential energies. C. its potential energy is always mgy plus or minus a constant. D. its potential energy cannot be defined.
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4 A non-conservative force… A. is a force for which a potential energy cannot be defined. B. is a force which does zero work when the object completes a closed path. C. is a force that remains constant. D. is a force that does not remain constant.
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5 The work done by the friction force… A. is always different from zero and negative. B. is always zero. C. cannot be written as a change in potential energy. D. is not discussed in the reading assignment.
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6 Office hours today • From 2 PM to 3 PM only.
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7
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8 Work by “conservative” forces The work done by conservative forces depends only on the initial and final points: We can define for each of these forces a potential energy Gravitational force: W y i ! y f ( ) = mgy i " mgy f Spring force: W x i ! x f ( ) = 1 2 kx i 2 " 1 2 kx f 2 U grav = mgy U el = 1 2 kx 2
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Properties of conservative forces • The work done by a conservative force can be expressed in terms of a potential energy: W grav y i ! y f ( ) = U grav y i ( ) " U grav y f ( ) = mgy i " mgy f W el x i ! x f ( ) = U el x i ( ) " U el x f ( ) = 1 2 kx i 2 " 1 2 kx f 2 The work done by a conservative force along a closed path is zero. W
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This note was uploaded on 10/05/2008 for the course PHY 121 taught by Professor Chamberlin during the Spring '08 term at ASU.

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PHY 121 Ch 7 Lecture - Lecture 12 Tuesday, February 26 1...

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