<p>what are you guys thinking for cutoff? My guess is 13ish…</p>
<p>How do you study for the semifinals then? Any good books you would recommend?</p>
<p>Hi guys, I already booked my plane ticket to physics camp this year at the University of Maryland. Does anyone know the best time/plane # for Kazakhstan as I plan to book my ticket for that as well.</p>
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<p>Solution:
We apply the law of motion directly, and assuming uniform density, the equation of motion is a = -4πGR^2 ρ/3r^2. Using the formula a = v dv/dr, we obtain a DE, which can be integrated to obtain |v| = sqrt(8πGR^2ρ/3 * (R/r - 1)).</p>
<p>The total time can now be found be integrating dt = dr/|v|, using a trigonometric substitution. We finally obtain T = sqrt(3π/32Gρ), so you were off by a factor of 1/sqrt(2). The integral that arose in the solution is definitely not something trivial, so I don’t think there is a simpler solution. </p>
<p>@fooiey
Use Kleppner, Purcell, and Irodov. Especially Irodov. </p>
<p>@mapletree7,</p>
<p>Although your solution is complicated, it is wrong. You cannot simply divide my answer by a factor of sqrt(2) arbitrarily and state that as the correct answer. Also your original equation of acceleration was not even dimensionally correct, so I believe you should go back to read Kleppner, Purcell, and Irodov.</p>
<p>Especially Irodov.</p>
<p>@mapletree7 I agree with greenbat23.</p>
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<p>Snarky responses? 
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<p>In any case, first, you should keep in mind that I did not simply divide your answer by 1/sqrt(2). My derivation produces the answer, T = sqrt(3π/32Gρ). I happened to note that your answer was off by that factor.</p>
<p>Forgive me, there was a slight typo in my equation of motion. I meant to say R^3, not R^2 (the volume of a sphere is of course proportional the the cube of the radius, but for some reason I didn’t catch that typo). That doesn’t change the validity of the final answer. </p>
<p>(If you still somehow insist that it is wrong, I invite you to look at F=ma 2013 #16). </p>
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<p>I noticed your post above. How are you so confident of IPhO?</p>
<p>@mapletree7 go on urbandictionary, and search “■■■■■”</p>
<p>^
Eh, I didn’t want to call him out as a ■■■■■, and I’m generally inexperienced with them in general, so . . … (and, trolls generally don’t respond to legitimate posts. . . apparently I was wrong). </p>
<p>@maple, you should be better educated about trolls like sciencechamp. anyways, i do believe my solution holds correct as there was nothing wrong with it.</p>
<p>@greenbat23 I’m not going to reread the problem and solution and all but mapletree’s problem is indeed identical to the 2013 F=ma #16, so that might help resolve any confusion.</p>
<p>.</p>
<p>when are results out?</p>
<p>should be later today. I was disappointed they weren’t out this morning.</p>
<p>@fooiey @greenbat23 Are you sure it’s today?</p>
<p>I thought so too, because of the “Feb 20th” on the exam packet. We can start discussing the test from today. </p>
<p>But last year, semifinalists were announced in March, weren’t they? <a href=“Semifinalists Announced for the 2013 US Physics Team.”>http://www.aapt.org/aboutaapt/2013PhysicsTeamSemifinalists.cfm</a></p>
<p>I hope I’m wrong, I can’t wait 10 more days. </p>
<p>Neither can I, I’ve been thinking about it since I took the test because I think I have a 50-50 chance of making it. So now can we discuss the questions? I’m really curious about 2 of the questions…</p>
<p>that’s just the official press release, last year they uploaded the semifinal list to their website on friday feb 22 (equivalent of tmrw)</p>
<p>I think we can start discussing.</p>
<p>Problem #16 of 2013? Can only be solved because it is a multiple choice problem.</p>
<p>The art of problem solving forum has all of the solutions to the previous f=ma tests</p>