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AAMC Self Assessment Physics, multiple questions

Discussion in 'MCAT Study Question Q&A' started by tshank, Sep 23, 2014.

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  1. tshank

    tshank 2+ Year Member

    107
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    Jun 3, 2014
    Physics #106

    What is the magnitude of the horizontal component of air resistance on the projectile at any point during the flight? (Note: Vx= horizontal speed.)

    Can someone explain the answer to this one? Their explanation is that ultimately PVx^2 is the same as (Vx/V)PV^2 Wouldn't that get you (P)(V)(Vx) not PVx^2? I thought those were different numbers since V and Vx are different values.







    PS #52

    Consecutive resonances occur at wavelengths of 8 m and 4.8 m in an organ pip closed at one end. What is the length of the organ pipe? (Note: Resonances occur at L = n(lambda)/4, where is the pipe lneght, lambda is the wavelength, and n = 1, 3, 5…..)

    [​IMG]
    Their explanation is attached (not their error of putting 0.8 m in the first line, should be 8 m) and it includes putting in n and n+2 for the equations. Can someone explain how or why we were supposed to put (n+2)(4.8) and not (n+2)(8)?

    Is the thinking, longer wavelength, shorter resonance? I have no idea.








    PS #56:

    Is there a simpler way to come to the same conclusion for this? They seem to complicate bernoulli's principle and give a convoluted answer when I think it is probably really simple.





    PS #61:

    Can anyone explain which equation we were supposed to use, I think I used to wrong one. It doesn't make sense to me that their explanation has why its (Pi/4) x (1.0 x 10^-2)^2 and not (1/2) x (1.0 x 10^-2/2)^2 I just don't see where the 5Pi comes from.
     

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  3. popopopop

    popopopop 5+ Year Member

    1,098
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    Dec 18, 2011
    DFW/Houston
    106.) I got it right, but I just went by the equation listed for the wind resistance since I know gravity is my Y component and X component horizontally isn't going to change. I guess initially V is the same as your Vx since you're shooting it horizontally and time of flight will only depend on gravity.

    52.) Don't know, but solving it your way gives a -5 for n if that means anything in this question.

    56.) I need to review my fluids again lol.

    61.) It's just volume flow rate = area x velocity. The math works out, you get 1.5 x 10^-5, but 5pi = 15~ so it's 10^-6. Tricky question if you aren't careful.
     
    tshank likes this.
  4. popopopop

    popopopop 5+ Year Member

    1,098
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    Dec 18, 2011
    DFW/Houston
    For 56, I rearranged P = 1/2pv^2 and solved for v. Apparently that's a thing? I never learned about these terms in class.

    http://en.wikipedia.org/wiki/Stagnation_pressure

    I got the answer right when I first did it and didn't hit guess, so I guess that's a method? Please correct me if I'm wrong. I certainly didn't solve it using AAMC's method.
     
  5. tshank

    tshank 2+ Year Member

    107
    16
    Jun 3, 2014
    Thanks for your responses. I appreciate it. Yeah, I think you actually did us their method, just simplified it though. You had P = pgh = 1/2pv^2 right? Same as their explanation essentially. Plug 5 in for h, isolate v, the p's cancel, and you are left w/ v = square root of 2gh. = 10.
    I think they applied the law of stagnation by saying the v at the top is negligible, and therefore zero, so good on you for doing it intuitively.

    61) After looking at it again, you're right. Just math. :( Shoot.

    106) It makes sense now after reading it again.

    52) Yes, doing it my way comes up with the wrong answer. But I don't know the rational to doing it the correct way. Any thoughts? Thanks.
     
  6. justadream

    justadream 5+ Year Member

    2,177
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    Apr 29, 2011
    #52

    Wavelength = 4L / n where n goes up by 2

    Shorter Wavelength is associated with larger n

    Thus:

    4.8 = 4L / n+2 ===> 4L = 4.8 (n+2)

    8 = 4L / n ===> 4L = 8n

    Set the two expression for 4L equal to each other:
    4.8(n+2) = 8n
    n = 3

    Plug n back into either equation:
    4L = 8*3
    4L = 24
    L = 6
     
  7. tshank

    tshank 2+ Year Member

    107
    16
    Jun 3, 2014
    Thanks for your post and explanation. Yes that makes sense. But how did you know that the shorter wavelength was associated with the smaller n? How do n and wavelength relate to each other within the pipe?
     
  8. justadream

    justadream 5+ Year Member

    2,177
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    Apr 29, 2011
    Wavelength = 4L/n for n, n+2, n+4, etc.

    Memorize that formula - that applies when you have 1 closed end

    As you increase n, wavelength decreases.
     
  9. tshank

    tshank 2+ Year Member

    107
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    Jun 3, 2014
    Thanks for that tip. But what does n represent? If the wavelength is the same, why does the n change? thanks!
     
  10. justadream

    justadream 5+ Year Member

    2,177
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    Apr 29, 2011
    I'm not sure how to explain it but n is like representing each possible iteration (maybe "scenario" would be a better word) of the wave.

    For the problem at hand:
    n = 1 = fundamental iteration
    n = 3 = second iteration.

    Look up some pictures of it and you'll understand it better.

    n doesn't really "change". It's just a different scenario.
     
  11. tshank

    tshank 2+ Year Member

    107
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    Jun 3, 2014
    Okay thanks. I thought the wavelength would stay the same in each scenario, but I was wrong. I understand it better now. Thanks for all the comments - it helped! :)
     

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