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Homework Help: Calculating air resistance homework

  1. Nov 29, 2006 #1
    1. The problem statement, all variables and given/known data

    I'm currently working on a lab which involves swinging a bung in the horizontal plane (just like a conical pendulum), however i'm a bit stumped at finding air resistance.

    The variables we know are the radius, the centripetal force and the velocity of the bung.

    3. The attempt at a solution

    In attempt to calculate the wind resistance, i did the following:

    W (non conservative) = delta KE +delta PE

    Force (of air resistance) x distance = KE (theoretical) - KE (Experimental)

    If i plotted KE theoretical and KE experimental on the y axis and radius on the x, there is a gap inbetween both series, a gap that gets wider as velocity increases. This makes sense because velocity is proportional to force. And here i yield difference of 0.5 to 0.7.

    HOWEVER, according to: Force (of air resistance) x distance = KE (theoretical) - KE (Experimental)

    if i divide both sides by distance (2 pi radius: the conical pendulumn travles in a circle) and when i plot the force of air resistance over radius, air ressistance seems to go down as r increased!! but this can't be the case because for some constant centripetal force, r is proportional to velocity SQUARED, and velocity is proportional to the force of air resistance.

    So what did i do wrong??

  2. jcsd
  3. Nov 29, 2006 #2


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    Homework Helper

    Welcome to PF mindboggling.

    Do your calculations indicate that the centripetal force is too large (for the given speed and radius)?

    How did you determine the magnitude of this force (weight hanging from the sting in the center)?
    Last edited: Nov 29, 2006
  4. Dec 8, 2006 #3
    centripetal force is always constant. The independent variable would be the radius, because in this lab we are trying to see how radius affects the speed for a given centripetal force.

    Yes, the magnitude of centripetal force is provided the weight hanging from the string in the center


    Many thanks
  5. Dec 8, 2006 #4


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    Homework Helper

    What I meant is that if you had the mass (of the bung) you could calculate it (the centipetal force) and compare it with the hanging weight. How do these two forces compare?
    Last edited: Dec 8, 2006
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