Transcript class slides for Chapter 7

7
Work and Energy
Lectures by James L. Pazun
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.
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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.
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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.
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Energy stored through elastic deformation - Figure 7.2
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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.
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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.
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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.
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Consider only parallel F and S – Figure 7.9
•Forces applied at angles must be resolved into
components.
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Applications of force and resultant work - Figure 7.10
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Sliding on a ramp - Figure 7.12 and 7.13
Please refer to the worked problem at the bottom of page 195
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Work done be several forces - Figure 7.14
•Refer to the worked example 7.3
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Work and energy related - Figure 7.15
• Unbalanced work causes kinematics.
• Refer to worked example 7.4.
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A pile driver application – Example 7.5
•Refer to
the
worked
example
on
pages
199-200.
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Work done by a variable force - Figure 7.17
• Depending on the
course, work by a
changing force is
sometimes considered.
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Energy stored by compression of springs - Example 7.6
•Energy may be stored in compressed springs.
•Refer to the worked example on pages 202-203.
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Non-mechanical work - Figure 7.22
• The force of
gravity can do work
and influence
energy
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A solved baseball problem - Figure 7.23
• Sports
solved by
energy
conservation.
•See worked
example on
page 205.
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Energy stored in spring displacement - Figure 7.25
• Elastic energy
stored in a spring
can be related to
motion.
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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.
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A spring safeguard for elevators – Figure 7.31
•Refer to
the worked
example
on page
212.
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Problems with non-conservative forces – Figure 7.32
• Refer
to
Example
7.12.
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Non-conservative Forces II - Figure 7.34
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Power considers work and time to do it - Figure 7.35
•Refer to the worked example on pages 217-218.
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