Friday, May 31, 2002
Announcements:
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Exam 1 scores for the multiple-choice section are available via the onyen
website: onyen.unc.edu
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A complete solution to Exam 1 will be posted on the library e-reserves
within the next few days.
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Summary of results from MC section:
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Average = Median = 49/75 = 65% (slightly better than last fall, but see
note below)
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Std. Dev. = 13%
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High = 72/75 = 96% (Way to go!!!)
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Low = 21/75 = 28% (expected score for random guessing is ~30%)
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Average for 14 questions and problems NOT on previous exams = 50%
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Average for 5 conceptual questions = 50%
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Average for 9 numerical questions = 49%
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Average for 11 questions and problems on previous exams = 86%
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Average for 5 conceptual questions = 95%
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Average for 6 numerical questions = 78%
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Conclusion: Many of you are better at memorizing answers than
understanding physics!
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The total Exam 1 score will include 10 points for the graphing question
and 15 points for the Santa problem.
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Keep in mind that this exam only counts for 10% of your overall course
grade, and the lab, class participation, and homework scores tend to boost
most students' scores. However, the average student improvement on
previous exams I have given is 0.2% and the maximum improvement was 13%
averaged over the course of the semester, so this first exam score is a
pretty good indicator of how you will do on future exams.
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Monday, June 3 is the last day to drop this course, so see me if you would
like to discuss your situation.
Chapter 5 - Work and Energy
Fundamental physics principle - Conservation of Energy: E = constant
in the Universe
Energy can be in various forms: (Ch. 5 focuses on mechanical
energy)
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Kinetic Energy: KE = 1/2mv2
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Gravitational Potential Energy: PEg = mgh
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Work: W = F*d
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Elastic Potential Energy: PEs = 1/2kx2
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Rotational Kinetic Energy: KEr = 1/2Iw2
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Electrostatic
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Electromagnetic (light, magnetic fields)
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Heat (from friction, air resistance)
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Sound (from collision, explosion)
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Chemical energy (battery)
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Nuclear
Problem-solving strategy: E(initial) = E(final) and define
energy forms specific to situation
Conservation of energy can be used to solve problems when F=ma is too
complicated or impossible:
Find the final speed of a child going down a slide that is 3 m high.
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Using Newton's second law: mgsin(theta) - umgcos(theta) = ma
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but F=ma can only be used for a flat (non-curved) slide so that acceleration
is constant
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Since the length of the slide was not given, we cannot even use F=ma.
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Energy: mgh = 1/2mv2 -> v = sqrt(2gh)
Easy!
Balls rolling down 3 different tracks - How do the final speeds compare?
Are average speeds the same?
Bowling ball demo - Is any energy dissipated?
How can you find the maximum speed of a pendulum if you only know its
length and maximum angle?
Work: W = F*d = Fcos(theta)d
(vector dot product)
Power: W/dt (rate that work is done)
Hints for HW5b:
P5.5 and P5.27: These two problems are routine applications of
conservation of energy. Use the examples in the book and the Study
Guide to help if needed.
P5.43: Use the definition of power: P = W/t and the units
of kg/s to find the rate of change of the gravitational potential energy.
P5.47: Use unit analysis and the definitions of power and speed
to help guide you to the correct answer. The units are a bit odd
for this problem since the problem gives an energy per step per kilogram.
Use this number to find the energy dissipated per step for the given mass
of the person, and then use this work/step and the power dissipated to
find
the time spent per step. The speed is then just the stide length
divided by this time.
P5.69: This problem is straightforward once you define the amount
that the bungee cord stretches (x) in terms of the initial height, the
final height, and the unstretched length of the cord. A complete
solution is available in the Student Study Guide.
Assignments:
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HW6a (due Monday)
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HW5b (due Tuesday)
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RWP5 - Super Dave Cannon (due Tuesday)