# ch5 - Chapter 5 Energy continued 1 Work and falling objects...

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Chapter 5 Energy ontinued continued 1

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Work and falling objects ystem a collection of objects A system is a collection of objects interacting via forces or processes at are internal to the ystem that are internal to the system 2
Potential Energy Energy associated with the position of an object Potential energy is a property of the system , not the object Scalar quantity SI units – J 3

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Gravitational Potential Energy Gravitational Potential Energy is the energy associated with the relative position of an object located near the Earth’s surface Objects interact with the arth through the earth through the gravitational force mgy PE g 4
Problem As an object is lowered into a deep hole in the surface of the Earth, which of the following statements must be assumed in regard to its potential energy? a) Increase b) Decrease ) emain constant c) Remain constant d) Cannot tell from the information given 5

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Work done by Gravity ) ( ) ( i f i f gravity y y mg PE PE PE W 6
eference Levels for Reference Levels for Gravitational Potential Energy A location where the gravitational potential energy is zero must be chosen hoice arbitrary - e choice is arbitrary the change in the PE is the important quantity etermined by convenience determined by convenience once the position is chosen, it must remain fixed 7

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Springs Found in everyday objects 8
Hooke’s Law Hooke’s Law gives the rce exerted by the force exerted by the spring = x F - k x F is the restoring force F is in the opposite pp direction of x k – spring constant SI Units N/m 9

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Potential Energy in a Spring Elastic Potential Energy elated to the work required to Related to the work required to compress a spring from its equilibrium position to some final, arbitrary, position x 2 1 kx PE 2 s 10
Spring Potential Energy, Example A) The spring is in equilibrium, neither stretched or compressed B) The spring is compressed, storing potential energy C) The block is released and the otential energy is potential energy is transformed to kinetic energy of the lock block 11

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ork- nergy Theorem Including Work Energy Theorem Including Gravity and a Spring W nc = KE - W c PE g is the gravitational potential energy ) ( ) ( ) ( , , , , i s f s i g f g i f nc PE PE PE PE KE KE W PE s is the elastic potential energy associated with a spring 12
Work-Energy Theorem, Extended In general, the work-energy theorem can be written as PE KE PE PE KE KE W i f i f nc ) ( ) ( PE will now be used to denote the total potential energy of the system 13

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Problem 5.20 When a 2.50- kg object is hung vertically on a certain light spring described by Hooke’s law, the pring stretches 2 76 cm spring stretches 2.76 cm.
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## This note was uploaded on 10/05/2009 for the course PHY 1603 taught by Professor Boudreaux during the Spring '08 term at The University of Texas at San Antonio- San Antonio.

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ch5 - Chapter 5 Energy continued 1 Work and falling objects...

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