Lectures by James L. Pazun 7 Work and Energy. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Goals for Chapter 7 Overview energy.

Lectures by James L. Pazun

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley

7 Work and Energy


Goals for Chapter 7

• Overview energy.

• Study work as defined in physics.

• Relate work to kinetic energy.

• Consider work done by a variable force.

• Study potential energy.

• Understand energy conservation.

• Include time and the relationship of work to power.


Introduction

• In previous chapters we studied motion

– Sometimes force and motion are not enough to solve a problem.

– We introduce energy as the next step.


An Overview of Energy

• Energy is conserved.

• Kinetic Energy describes motion and relates to the mass of the object and it’s velocity squared.

• Energy on earth originates from the sun.

• Energy on earth is stored thermally and chemically.

• Chemical energy is released by metabolism.

• Energy is stored as potential energy in object height and mass and also through elastic deformation.

• Energy can be dissipated as heat and noise.


Energy stored through elastic deformation - Figure 7.2


A study of energy transformation - Figure 7.4

• This transformation begins as elastic potential energy in the elastomer. It then becomes kinetic energy as the projectile flies upward. During the upward flight, kinetic energy becomes potential until at the top of the flight, all the energy is potential. Finally, the stored potential energy changes back to kinetic energy as the projectile falls.


Energy can be “lost” as heat. - Figure 7.6

• Energy can be dissipated by heat (motion transferred at the molecular level. This is referred to as dissipation.


What is “work” as defined in Physics? – Figure 7.8

•Formally, work is the product of a constant force F through a parallel displacement s.


Consider only parallel F and S – Figure 7.9

•Forces applied at angles must be resolved into components.


Applications of force and resultant work - Figure 7.10


Sliding on a ramp - Figure 7.12 and 7.13

Please refer to the worked problem at the bottom of page 195


Work done be several forces - Figure 7.14

•Refer to the worked example 7.3


Work and energy related - Figure 7.15

• Unbalanced work causes kinematics.

• Refer to worked example 7.4.


A pile driver application – Example 7.5

•Refer to the worked example on pages 199-200.


Work done by a variable force - Figure 7.17

• Depending on the course, work by a changing force is sometimes considered.


Energy stored by compression of springs - Example 7.6

•Energy may be stored in compressed springs.



Non-mechanical work - Figure 7.22

• The force of gravity can do work and influence energy


A solved baseball problem - Figure 7.23

• Sports solved by energy conservation.

•See worked example on page 205.


Energy stored in spring displacement - Figure 7.25

• Elastic energy stored in a spring can be related to motion.


Conversion and conservation – Figures 7.27, 7.28

•As kinetic and potential energy are interconverted, dynamics of the system may be solved.

and

Refer to the worked examples on page 209.


A spring safeguard for elevators – Figure 7.31

•Refer to the worked example on page 212.


Problems with non-conservative forces – Figure 7.32

• Refer to Example 7.12.


Non-conservative Forces II - Figure 7.34


Power considers work and time to do it - Figure 7.35


Lectures by James L. Pazun 7 Work and Energy. Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Goals for Chapter 7 Overview energy.

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