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    Barbarian in the Playground
     
    Don Beegles's Avatar

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    Default Physics Question

    It feels wierd that my first post after my long silence (which wasn't over anything important, if anyone had wondered at all; I merely had no internet for a while, was at camp, and I'd rather talk to my girlfriend than most) is to ask a homework question, but here it goes anyway;

    I'm taking AP physics this year, and the summer assignment is to read a chapter of the book and answer the questions at the end. I think I got the general idea on most of them, which is all he expects, but I have no idea on the very last one. It is "Use conservation of momentum to explain why photons emitted by hydrogen atoms have slightly less energy than predicted by Eq.27-10." Eq.27-10 is Bohr's hf=Eu-El.

    I really have no idea at all, and was hoping that someone here would be able to give some clue.
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    Default Re: Physics Question

    Physics is all fiction.
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    Default Re: Physics Question

    I'm not at all certain, but here's my guess. Photons do not have mass (m=0), but they do have momentum. In conservation of momentum, you have several points where the calculation is based on mass * vector. Setting mass at zero for those points may give some odd results.
    Last edited by Telonius; 2007-08-16 at 02:00 PM.
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    Dwarf in the Playground
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    Default Re: Physics Question

    Been a long time since I did physics, but think of it this way:

    1. Assume that the momentum of the Hydrogen starting out is X.
    2. You end up with two particles with their own momentums.
    3. Momentum must be conserved.
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    Default Re: Physics Question

    Quote Originally Posted by mudbunny View Post
    Been a long time since I did physics, but think of it this way:

    1. Assume that the momentum of the Hydrogen starting out is P (symbol changed from X in the original post).
    2. You end up with two particles with their own momentums.
    3. Momentum must be conserved.
    Yeah, that's basically it. You can even set P = 0 so that you are in the rest frame of the hydrogen atom.

    The transition of an electron into a lower energy state releases a certain amount of energy.

    Most of that energy goes into the photon, but not all of it, because of conservation of momentum. Essentially, there's a tiny bit of recoil that kicks the H atom into motion in the opposite direction of the photon's motion (EDIT -- Which is necessary to keep the total momentum = 0). This energy has to come from the energy released by the electron's transition, so the resultant photon has a tiny bit less of total energy and momentum than you would predict from the formula.
    Last edited by Ted_Stryker; 2007-08-16 at 02:14 PM.

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    Don Beegles's Avatar

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    Default Re: Physics Question

    Thanks a lot everyone.

    That's simpler than I thought. It's actually almost exactly what I was thinking, but it didn't seem right to me. Apparently it is and I was overthinking it.
    Cardinal Ximinez by Mephibosheth
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    And yes, that is pretty much exactly what she meant. Don Beegles FTW!
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    Default Re: Physics Question

    Quote Originally Posted by PlatinumJester View Post
    Physics is all fiction.
    Truer words have never been spoken.
    In fair Verona~

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    Default Re: Physics Question

    Quote Originally Posted by PlatinumJester View Post
    Physics is all fiction.
    Any Ninja could tell you that.
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    Default Re: Physics Question

    Quote Originally Posted by PlatinumJester View Post
    Physics is all fiction friction.
    An easy mistake to make.

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    Default Re: Physics Question

    No mistake. I won't believe in Gravity until I see it with my own eyes. Neuton was such lier. He probably got crapped on by a bird BUT said it was an apple to stop himself from looking stupid.

    Friction is also made up as is Radiation, electrical current and Al Gore.
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    Ogre in the Playground
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    Default Re: Physics Question

    Tell you what if you don't belive in gravity, just lean out of the window of a tall building, say to about 90 degrees... see how you feel about it then shall we?
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    Bugbear in the Playground
     
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    Default Re: Physics Question

    just write down that a wizard did it, you should get credit for trying.

    also, gravity sucks.

    gravity was the science cover of magic, see we're all pulled to the earth by magic, thats why we can't fly, it's just been around so long everyones accepted it.
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