Lectures by James L. Pazun 7 Work and Energy

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

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

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

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Energy stored through elastic deformation - Figure 7.2

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

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

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

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Consider only parallel F and S – Figure 7.9 Forces applied at angles must be resolved into components.

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Applications of force and resultant work - Figure 7.10

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Sliding on a ramp - Figure 7.12 and 7.13 Please refer to the worked problem at the bottom of page 195

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Work done be several forces - Figure 7.14 Refer to the worked example 7.3

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Work and energy related - Figure 7.15 Unbalanced work causes kinematics. Refer to worked example 7.4.

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley A pile driver application – Example 7.5 Refer to the worked example on pages

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Work done by a variable force - Figure 7.17 Depending on the course, work by a changing force is sometimes considered.

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Energy stored by compression of springs - Example 7.6 Energy may be stored in compressed springs. Refer to the worked example on pages

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Non-mechanical work - Figure 7.22 The force of gravity can do work and influence energy

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley A solved baseball problem - Figure 7.23 Sports solved by energy conservation. See worked example on page 205.

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Energy stored in spring displacement - Figure 7.25 Elastic energy stored in a spring can be related to motion.

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

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley A spring safeguard for elevators – Figure 7.31 Refer to the worked example on page 212.

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Problems with non-conservative forces – Figure 7.32 Refer to Example 7.12.

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Non-conservative Forces II - Figure 7.34

Copyright © 2012 Pearson Education, Inc. publishing as Addison-Wesley Power considers work and time to do it - Figure 7.35 Refer to the worked example on pages