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Engery- Falling Coffee Filter Problem

  1. Oct 11, 2007 #1
    You drop a single coffee filter of mass 1.4 grams from a very tall building, and it takes 47 seconds to reach the ground. In a small fraction of that time the coffee filter reached terminal speed.


    (a) What was the upward force of the air resistance while the coffee filter was falling at terminal speed?
    Fair = N

    b) Next you drop a stack of 3 of these coffee filters. What was the upward force of the air resistance while this stack of coffee filter was falling at terminal speed?
    Fair = N


    (c) Again assuming again that the stack reaches terminal speed very quickly, about how long will the stack of coffee filters take to hit the ground? (Hint: Consider the relation between speed and the force of air resistance.)
    Fall time is approximately ___s

    Where do i need to start to begin this problem?? Thanks for the help!! :-)
     
    Last edited: Oct 11, 2007
  2. jcsd
  3. Oct 11, 2007 #2

    learningphysics

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    What is the net force on the filter at terminal velocity?
     
  4. Oct 11, 2007 #3
    zero
     
  5. Oct 11, 2007 #4
    because there isn't any more change in momentum
     
  6. Oct 11, 2007 #5

    learningphysics

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    exactly. So you have the upward air resistance force and downward gravitational force acting on the object... so what is the air resistance force equal to at terminal velocity?
     
  7. Oct 11, 2007 #6
    ahhh...i got it. mg = Fair. Now I'm having issues with part c? would you just divide 47 by 3?
     
  8. Oct 11, 2007 #7

    learningphysics

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    What is the equation of air resistance force in terms of velocity?

    What is the terminal velocity in the case of the single filter... using the case of the single filter, what is the height?

    What is the terminal velocity in the case of 3 filters... what is the time using the height calculated previously?
     
  9. Oct 11, 2007 #8
    In the book they give us an equation for approximate air resistance?
     
  10. Oct 11, 2007 #9

    learningphysics

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    what's the equation?
     
  11. Mar 5, 2008 #10
    can someone elaborate on how to get c? the previous posts don't help at all
     
  12. Mar 5, 2008 #11

    dynamicsolo

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    What learningphysics asked you for is reasonable. What relationship have they given you to work with, giving some proportionality between the force of air resistance and the speed of the object through the air. Is it F = k·v? F = k·(v^2)? The constant k is the "coefficient of drag", which you will be able to eliminate when you compare the terminal velocities for parts (b) and (c). You know what the resistance force equals at terminal velocity in both cases, so a ratio of the forces will give you a ratio involving the two different terminal velocities. (You don't need to know the height the filters fall through. Just call it H, since it will also cancel out in your comparison ratio.)
     
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