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Need help with a physics problem. massive karma for good ideas, or ideally the solution!

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chicken
Posts: 6843 - Karma: 2,123
So here is the question and all I have done so far to figure it out

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the s subscript means at the critical point. I know that at the c.p. those two partial derivatives must be zero. so now how the heck do I solve for a and b to produce those answers?

May 30 2012 8:37PM
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chicken
Posts: 6843 - Karma: 2,123
and yes, NS was my last attempt. Tried google, yahoo answers, cramster, my friends. need this tonight! everywhere I have searched gets to the point where I am, and then "magically" arrives at these numbers, so it must be trivial. I just can't see how.
May 30 2012 8:40PM
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AcidKneeDrop
Posts: 890 - Karma: 279
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May 30 2012 8:41PM
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the.cheat
Posts: 2859 - Karma: 171
came in here expecting to help with high school kinematics, lolnope, good luck.
May 30 2012 8:46PM
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Casper.
Posts: 622 - Karma: 65
You have a good name to help someone "cheat" on their work too.. too bad!
May 30 2012 8:47PM
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abathinggrape
Posts: 1228 - Karma: 236
karma isnt enough, money is
May 30 2012 8:47PM
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stickski
Posts: 1938 - Karma: 1,529
same. i could've helped but no...
May 30 2012 8:48PM
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Steeze_Minister
Posts: 1432 - Karma: 240
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but in reality I came here expecting a high school level physics hoping I could help, so good luck
May 30 2012 8:48PM
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rareison
Posts: 10263 - Karma: 219
jesus christ, and I thought grade 9 was hard. fuck my life.
May 30 2012 8:53PM
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.Hugo.
Posts: 17360 - Karma: 28,484
English please? But in all seriousness I fucking hate physics, have to take a full year of it next year and i'm not looking forward to it
May 30 2012 9:01PM
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chicken
Posts: 6843 - Karma: 2,123
how much?

May 30 2012 9:06PM
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chicken
Posts: 6843 - Karma: 2,123
I know there are physics students on NS because I have seen you in other homework help threads.

I wouldn't consider this cheating as I will obviously not submit it unless I fully understand it! I just really need to know the answer.

also I gave a bit of karma to everyone who replied just for bumping.
May 30 2012 9:08PM
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chicken
Posts: 6843 - Karma: 2,123
if the physics is throwing you off, just essentially I have the first three equations to work with, with the second and third equal to zero.

It's the math I don't get
May 30 2012 9:10PM
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45.
Posts: 3164 - Karma: 170
Two equations - Two unknown's.

Create a system by isolating the desired variables the best you can. Or - if your lazy, type the system into a TI-89 and solve it.
May 30 2012 9:11PM
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the.cheat
Posts: 2859 - Karma: 171
May 30 2012 9:12PM
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45.
Posts: 3164 - Karma: 170
Forgot to mention - you will need to sub in the values for Tc and Vc and whatever else so that your equation is entirely in terms of R, a, and b. With any amount of mathematical luck - the R values will end up canceling out.

If not - give up and cry.
May 30 2012 9:14PM
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chicken
Posts: 6843 - Karma: 2,123
I can't sub in the values of Tc and Vc because that is what I am looking for. I guess I wasnt that clear but I need Tc vc and Pc in terms of a,b and R. The answer is up there, I just cant get from start to finish!

+K
May 30 2012 9:30PM
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chicken
Posts: 6843 - Karma: 2,123
I'm pretty sure this is the real world... I know I will probably have to use it at least once in the "real world" when I write my exam. Even if I never need to use it again after that, it never hurts to learn something new.

If people didn't have curiosity and try to figure out this kinda stuff, we would be back in the stone age still. We certainly wouldn't be able to have a conversation on the internet or drive to a ski hill, or run a chairlift, without this stuff!
May 30 2012 9:48PM
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TheClap.
Posts: 21664 - Karma: 4,140

May 30 2012 9:54PM
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45.
Posts: 3164 - Karma: 170
Well my suggestion still stands - rather set both partials equal to each other and sub in the value of Tc and solve for Vc. Then vice versa, that will get you those two terms entirely in terms of a,b, and R. That would be where I would start anyways.
May 30 2012 9:54PM
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crab_pretzel
Posts: 3919 - Karma: 622
haha couldnt put it better myself. and i would title this thread calc problem not physics, even if it is for physics class, just because its more relavent to that
May 30 2012 9:56PM
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funkmaster
Posts: 617 - Karma: 1,876
this, once you get Tc and Vc in terms of a, b and R you can sub said expressions and solve for Pc.
May 30 2012 10:59PM
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chicken
Posts: 6843 - Karma: 2,123
GOT IT. Thanks everyone, especially 45 and funkmaster! I had the same idea as you guys, but gave up when it got messy. I tried it again after that and got it right away.

If anyone wants to see it I will post it, it's about two pages of math though.

May 31 2012 12:37AM
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chicken
Posts: 6843 - Karma: 2,123
I guess it's useless in the real world, just like calculus and linear algebra.

I wonder why they even bother teaching it to every science and engineering student on the planet...
May 31 2012 12:41AM
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