| | Lessons from my physics finalI've been requested to write more on here. So here's something.
Just finished my physics final in Classical Mechanics.
Professor Mon makes the tests pretty easy--much easier than the homework. In fact, I got a 100 on the last test, of which I am very proud, even though the questions weren't very tough. Because of that, I was not worried about this test, so I did not study, and I realized by this morning I had really not prepared myself for it adequately.
I did some minor brushing up on all the old concepts (it was comprehensive) and went in just ready to take what was coming.
Question 4 was about a planet orbiting a sun, at a given linear velocity v, and therefore, a momentum p=mv.
Part (a) asked for the magnitude of the linear momentum, which I by some happenstance remembered to be
This is simple, it means ℓ=rpsinθ=rmv, and sinθ=1 since θ=90.
I got this quickly and easily, and this momentum propelled me through the rest of the problem quickly. Part (b) was something I don't remember, and then part (c) was just asking for the magnitude of the torque the sun exerts on the planet. I recalled from somewhere that
and hence,
Once again, I just automatically wrote T=rF (he allowed us to just give the force as F) and moved on. Happy me, problem done.
I finished all the questions, and decided, since I had two hours left, to just look it over again. This is almost unheard of for me. I just don't check my tests. It's never been in my nature.
I came again to problem 4, and was looking over my clean, easy work. Then something about part (c) looked wrong to me. Perhaps the ambiguity of the torque's direction leapt out at me, I'm not sure. But I looked at it closer, and realized the mistake: r and F are parallel, so θ=0! An easy mistake: after the first part, I was just in the mindset of thinking the angle would be 90. I corrected my answer and moved on. I'm sure other people got it wrong, it was an easy mistake and I'm sure an intentional trap. I was lucky to catch it, and now feel rather heartened about it.
That's all I had to say. This was fun to write, because of all the math formatting.
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| | Posted 12/10/2007 2:13 PM - 1 comments
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