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PHY 201
Your 'question form' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
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Please see the next box for my responses to your responses about PHY 201 Final Exam. I am not asking for a different grade, because obviously I am perfectly content with the Course Grade. I am pretty sure on the first problem below, that I solved it correctly, just as you stated. Please see the other two. I am just curious as I want to make sure I understand my mistakes, especially the one dealing with the PE, KE, and TE of the pendulum. I may have totally missed that through the course. Thanks so much.
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Very good work, but consider the following:
Coasting down a hill I reach a certain speed. The hill is not assumed to have a uniform slope, so it is inappropriate to assume uniform acceleration. The equations of uniformly accelerated motion therefore do not apply. However given the speed at the bottom of the hill, and the speed attained coasting down the second half of the hill, we can find the KE change for the entire hill, and that for the second half. This allows us to figure out the KE change in the first half, which can be set equal to the PE loss in order to figure out the elevation change on the first half.
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I am pretty confident that this is exactly how if worked out this problem. If I did have any equations from uniformly accelerated motion, I may have started working it out that way but I then worked it through with the changes in KE. I don’t think I did this though. I also solved for the height just like you said setting the KE change equal to the PE loss.
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To find the PE and KE of a pendulum at the halfway point between maximum displacement and equilibrium, knowing the total energy, we reason as follows: Total energy = 1/2 k A^2; at the halfway point the PE is 1/2 k ( 1/2 A)^2 = 1/8 k A^2, which is 1/4 of the total energy. The kinetic energy at this point is therefore 3/4 of the total energy.
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The 1/8 k A^2 would be the PE halfway between equilibrium and A, is this correct. I thought that the mass would have no KE at A, and therefore all TE would be PE. And at equilibrium, it will all be KE and no PE since x=0 at equilibrium. Am I wrong in the above, or did I read the problem wrong and it asked to compare these quantities at a point between equilibrium and extreme?
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The problem asked a number of questions, one of which was the PE and KE at the point halfway to equilibrium. I believe you answered everything correctly, except for that one part of the question.
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In an elastic collision kinetic energy is the same after as before the collision. In any other collision in which no internal or external source of energy is converted, kinetic energy is not conserved so the KE after collision is less than before collision. So if total KE goes from 100 J to 10 J, this is consistent with the nature of a nonelastic collision.
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I was really unsure on this one but this helps. Thanks.
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I don't have the test at this location but I'll take another look, especially on the problem about the hill. I'll get back to you on that.
Check my note on the problem about the pendulum.
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