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AAMC Self Assessment Physics, multiple questions

Discussion in 'MCAT Study Question Q&A' started by tshank, 09.23.14.

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  1. tshank

    tshank 2+ Year Member

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    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 2+ Year Member

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    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.
     
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  4. popopopop

    popopopop 2+ Year Member

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    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

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    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

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    #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

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    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

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    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

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    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

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    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

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    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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