(1) Question 49: The equation v2 = Agr was meant to be v2 = Agr*cos(theta_d). In either case, A is just a constant, but I much prefer the latter equation. However, you can give the answer assuming either equation, and I will grade it with two possible correct answers.
(2) Questions 90, 91, 92: Please consider the monkey at the same height of the center of the rotor (the origin of the coordinate system). I.e., the monkey is hugging the blade just so that his center of mass is at the same height as the center of the rotor. So, no potential energy (due to gravity) to consider for these two particular questions, since it is zero.
(3) Question 37: This question asks whether or not the direction of force n that you derive from solving Newton's 2nd law equation (radial components only) is physically correct. I.e., OK means "physically plausible." It is a choice problem between "is" and "is not".
(4) Misconception Warning!!!: I heard that somehow there is a notion floating around that, in part 1 of problem 1, the bug can stay on the sphere up to 90 degrees of rotation. This is incorrect! The whole point of this problem is to show that this is not the case, and the bug leaves the surface much much earlier on. I might point out to you that, if you are still confused about the answer of this problem, you shoul read problem 7.62 of the textbook carefully. Also, you may have taken notes when I went over this problem in class. Please master those notes.
Wednesday, November 26, 2008
Other comments on extra credit problems
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A student asked this question: "I have a question about part 1...is the centripetal accel. it is talking about the actual, or the *required*?? Logically speaking, there is no centripetal accel., right? as it is just a function of the gravity pulling the bug down into a circle path."
Here is my answer: I think there may be a confusion here. Centripetal acceleration is real, physically or logically. For any circular motion, the radial components of Newton's equation tell you what forces add up to give the centripetal acceleration. Yes, in this case, the gravity is the important part of what gives rise to the centripetal acceleration.
hey sam
i was wondering. if we get a wrong answer for one of the questions, will the subsequent answers be wrong too?
For answering questions about speed, do I use the term "increase" or "accelerate" for a speed that is getting larger? I am just confused by the fact that both accelerate and increase are options. I thought they meant the same thing.
crazyh20azn: Each question has one right answer, and only one. An exception is 49 (see above).
J. Quinn: In that case, you should choose "increase" rather than "accelerate". It may be OK to say "speed accelerates" in a colloquial sentence. However, in this course, "accelerate" means "change/increase in velocity" as in "car accelerates."
A student question: "In doing the extra credit problems i am confused on what anti-parallel means in describing the relationship of two vectors. Should i take it to mean that they are parallel but in opposite directions or that they are not parallel and thus perpendicular?"
My answer: for two anti-parallel vectors, the angle between them is 180 degrees.
I think it's pretty much the same question as J. Quinn but worded a bit differently. Would you say you accelerate/decelerate angular motion or increase/decrease angular motion? I thought the first one since increase/decrease motion seems a bit ambiguous.
Yes, Kevin, that is correct.
hey sam,
im confused for problem 14, im not very clear on the question. what is "it"? and by looking at it, isnt the angle the angle theta that u gave us?
crazyh20azn: "it" means the force vector in the previous question. how do you measure the angle between two vectors? first, use the same origin for the two vectors, and ...
Sam,
I realize this may be difficult because everyone had different numbers, but is there anyway we can see what the correct answers were to the extra credit problems before the final to help study? I think that would be very helpful.
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