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Thread: Need help with science/math schoolwork? Post here!

  1. #1
    chris_uk_83's Avatar Physicist
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    Icon2 Need help with science/math schoolwork? Post here!

    Do YOU have problems with your physics homework? Is it driving you mad? Do you fail to see precisely why nothing can travel faster than the speed of light?

    Don't worry any more, Chris is here to help! Simply post your problems and I'll do my best to explain the answers to you.

    I dare say Fish might have a go at the biology ones too, and I'm sure there are chemists lurking somewhere.

    If I've helped you, rep me. I live for rep.

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    Simetrical's Avatar Former Chief Technician
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    Default Re: Does anyone need physics help?

    I do physics and math tutoring (although not very advanced, in the case of physics). Hmm . . . let's pin this and make it a general tutoring thread.
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    therussian's Avatar Use your imagination
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    Default Re: Need help with science/math schoolwork? Post here!

    Physics and maths here too, along with literature analysis and all that.

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    chris_uk_83's Avatar Physicist
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    Default Re: Need help with science/math schoolwork? Post here!

    I'd just like to say,

    [move]Yay! I got a thread stickied by accident[/move]

    Admins, feel free to remove this completely frivolous post.

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    Default Re: Need help with science/math schoolwork? Post here!

    I can help with Bio, particularly cell bio, human physiology/pathology, pharmacology, and some (medically relevant) biochem. Right now it seems we have more teachers than students, though that's not necessarily a bad thing!
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    TheKwas's Avatar Civitate
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    Default Re: Need help with science/math schoolwork? Post here!

    Let me post a math question, but I don't know how to work fancy math symbols so I'm not sure how this will work out.

    Calculate:

    Infitnity sign
    S ............. dx
    1 ............ ____
    ............... x^3


    S=anti-derivative sign.

    The answer is 1/2 but if anyone can show me how to get to that solution step by step I will spam all your posts with +rep.
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    4) The most formidable handicap for a creator would be non-existence.
    5) Therefore if we suppose that the universe is the product of an existent creator we can conceive a greater being — namely, one who created everything while not existing.
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    therussian's Avatar Use your imagination
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    Default Re: Need help with science/math schoolwork? Post here!

    well, first you integrate it: S (x)^-3 dx, which would become (-1/2)x^-2. Then you plug in infinity and one into the antiderivative. {-1/2x^2] from 1 (a) to infinity (b). F(b) - F(a), and you get 1/2.

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    TheKwas's Avatar Civitate
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    Default Re: Need help with science/math schoolwork? Post here!

    S (x)^-3 dx, which would become (-1/2)x^-2.
    This is likely the simplist part, but I'm pretty sure this is what I'm having trouble with. Can you break this down any further?
    1) The creation of the world is the most marvelous achievement imaginable.
    2) The merit of an achievement is the product of (a) its intrinsic quality, and (b) the ability of its creator.
    3) The greater the disability (or handicap) of the creator, the more impressive the achievement.
    4) The most formidable handicap for a creator would be non-existence.
    5) Therefore if we suppose that the universe is the product of an existent creator we can conceive a greater being — namely, one who created everything while not existing.
    6) Therefore, God does not exist.


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    therussian's Avatar Use your imagination
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    Default Re: Need help with science/math schoolwork? Post here!

    well, dx/x^3 is equivalent to (x^-3)dx, correct? To integrate these types of terms (or whatever their called), you do the power rule, where you raise the term of an exponent by one (which in this case would become the -2nd power), and you also divide by that number (-2), which gets you (-1/2)x^-2.
    Last edited by therussian; April 23, 2007 at 07:11 PM.

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    TheKwas's Avatar Civitate
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    Default Re: Need help with science/math schoolwork? Post here!

    lewl, I'm a retard.

    +rep.
    1) The creation of the world is the most marvelous achievement imaginable.
    2) The merit of an achievement is the product of (a) its intrinsic quality, and (b) the ability of its creator.
    3) The greater the disability (or handicap) of the creator, the more impressive the achievement.
    4) The most formidable handicap for a creator would be non-existence.
    5) Therefore if we suppose that the universe is the product of an existent creator we can conceive a greater being — namely, one who created everything while not existing.
    6) Therefore, God does not exist.


    Garbarsardar's love child, and the only child he loves. ^-^

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    therussian's Avatar Use your imagination
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    Default Re: Need help with science/math schoolwork? Post here!

    No problem


    NEXT!

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  12. #12
    chris_uk_83's Avatar Physicist
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    Default Re: Need help with science/math schoolwork? Post here!

    I just looked at this thread again, and I had awful trouble reading that maths. Does anyone know of an easier way of posting mathematical equations? I'd suggest using MS Word's equation editor (which most people probably have) and attaching the word file. There must be an easier way than this though.

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  13. #13

    Default Re: Need help with science/math schoolwork? Post here!

    Probably,Chris.But I am going to use Microsoft Math which I got with Encarta 2007...

    Except in this one...



    Could somebody help me in determining the time period of oscillations,if a particle with -q charge is displaced from the origin to a point,x m away from the origin in a direction perpendicular to abscissa,already under the influence of two charges,+q each separated by a distance of 2a m?The given condition is that a>>x.
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    Default Re: Need help with science/math schoolwork? Post here!

    h
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    chris_uk_83's Avatar Physicist
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    Default Re: Need help with science/math schoolwork? Post here!

    At first glance, that doesn't oscillate.

    The forces from the two charges are equal and opposite so they effectively cancel out entirely, resulting in no net force on the negative charge in the middle.

    I have a feeling you mean this:



    which I will look up when I get home. But for the time being think about the repulsive force increasing as you approach an opposite charge and decreasing as you move away from an opposite charge.

    electric force is given by:



    where q1 and q2 are the charges of each charge (1 and -1) and r is their separation. Remember to treat each pair seperately, you can't incorporate all three into the same force equation easily, so work out the seperate forces and add them together.

    EDIT: also think about Simple Harmonic Motion, using the force you obtain from adding together the (seperate) forces from the two positive charges acting on the central negative charge.
    Last edited by chris_uk_83; April 24, 2007 at 04:13 PM.

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    Default Re: Need help with science/math schoolwork? Post here!

    @Chris:
    Well,not really.I meant that diagram actually.Simple Harmonic Motion won't be possible in the case you are describing.When the particle with -q charge is displaced towards one of the +q charged particle,it gets displaced towards the nearer charge and won't show any oscillations,thereby neglecting the smaller force exerted by the distant particle.Hence,the simple harmonic motion has to be on the vertical axis instead of horizontal with the mean position at (0,0,0).But I couldn't relate the force exerted with the time period which could supposedly be calculated by the value of angular frequency(ω).And most importantly,the forces won't cancel each other as they are not in opposite directions.
    Last edited by Banned; April 25, 2007 at 03:44 AM.
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    chris_uk_83's Avatar Physicist
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    Default Re: Need help with science/math schoolwork? Post here!

    Crap, you're right I completely ignored the fact that the centre charge is negative, and NOT positive. If it were +ve I'd be right. I really ought to rtfq!

    Right, new thought train:
    the horizontal forces do cancel out (but the vertical forces are additive). That's why the -ve charge only moves up and down and not left and right. Therefore you can work out the y component of the force acting on the -ve charge at varying values of x (think trigonometry y = F sin(angle) ). This force will change with x since the angle will change with x and so will the distance between the -ve and the two +ve charges.

    Still working on relating force to SHM, but you can easily get the period from the angular frequency since ω=2(pi) f. Where f is frequency (in revolutions per second or Hz). The period T is then 1/f.

    EDIT: It might help me to find the answer if you tell me what level this question is pitched at (which year of a degree etc.)
    Last edited by chris_uk_83; April 25, 2007 at 06:15 AM.

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  18. #18

    Default Re: Need help with science/math schoolwork? Post here!

    Quote Originally Posted by chris_uk_83 View Post
    Therefore you can work out the y component of the force acting on the -ve charge at varying values of x (think trigonometry y = F sin(angle) ).
    Yeah,but the angle isn't known whose absence is the only hurdle in solving this problem.

    And I got this question from a random book containing questions of Physics with no answers...lol!
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  19. #19
    chris_uk_83's Avatar Physicist
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    Default Re: Need help with science/math schoolwork? Post here!

    EDIT: you can work out the angle because you know x and you know a.

    There you go, got it for you. Thanks to JoAuSc from physicsforums.com.

    The force from each charge has two components. Using symmetry arguments you can see the vertical components cancel out, and that the horizontal components (the y-components) of the two forces from the two charges add up. Let psi be the angle between the diagonal and the vertical axis (it seems to be a more convenient angle to work from than theta).

    To find the y-components of the forces, we need to make two approximations: 1.) because the distance y is small, the change in the distance r between q and each Q will not change much from d/2; 2.) the force component in the y-direction is F*sin(psi), and for small angles sin(psi) = psi. (Zoom in on the portion of the sin(x) vs. x curve around the origin and you'll see it resembles a line with slope 1 crossing the origin). So F*sin(psi) ~ F*psi. Don't forget to change psi to the distance y that it corresponds to.

    The equation for simple harmonic motion is

    m*a - (restoring force) = 0

    where the restoring force is proportional to x (else it wouldn't be "simple"). If you do the algebra with F*psi you'll find you get some expression that's proportional to y. Let that expression be k. According to formulas about SHM, the angular velocity omega is equal to sqrt(m/k), and the period is equal to 2pi/omega. Do the algebra, and you should end up with the right answer.

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  20. #20

    Default Re: Need help with science/math schoolwork? Post here!

    Quote Originally Posted by chris_uk_83 View Post
    EDIT: you can work out the angle because you know x and you know a.
    You mean something like Φ=sinˉ¹[a/(a²+x²)½] or Φ=cosˉ¹[x/(a²+x²)½],as a measure for the angle unknown?But I doubt that the real answer consists of either sin or cos.Is there a different way to move about,except assuming an angle?
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