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Wind speed question

  1. Sep 26, 2004 #1
    i have calculated volume and weight from different size hailstones.
    i now have to calculate the wind speed needed to keep a hailstone of each size aloft.
    i am stuck on which equation to use...
    should i solve using
    D = 1/2(C*rho*A*v^2) ?

    for rho do i plug in the density of air or of each hailstone?

    i can only get as far as plugging in values for
    (drag coefficient) C=.5 and A for each respective hailstone.

    thanks!
     
  2. jcsd
  3. Sep 26, 2004 #2

    Tide

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    That would be the density of air but I don't see how the wind will keep a hailstone aloft unless the wind is directed upward.
     
  4. Sep 26, 2004 #3
    we were given a hint: your calculations will involve the weight of the hailstone and the upward force of wind on the hailstone.
     
  5. Sep 26, 2004 #4

    Tide

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    Okay, so it's an updraft!

    You can calcuate the drag force on the hailstone and compare it with the weight of the hailstone.
     
  6. Sep 26, 2004 #5
    i have an answer D = 1.40

    the units that don't cancel out are gram/sec

    is my answer for D
    D = 1.40 grams/second?

    i can't find a unit for D anywhere...is this acceptable?
     
  7. Sep 26, 2004 #6

    Tide

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    Use
    [tex]mg = \frac {1}{2} C \rho A v^2[/tex]
    to find the speed. The units should work out.
     
  8. Sep 26, 2004 #7
    thank you SOOOOOOOOOO much!
     
  9. Sep 27, 2004 #8
    would that answer be the wind speed of the terminal velocity of the hailstone?
    thanks!
     
  10. Sep 27, 2004 #9

    Tide

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    Exactly! :smile:
     
  11. Sep 27, 2004 #10
    this is really confusing...the 1st question was "for each size hailstone, calculate the wind speed needed to keep a hailstone of that size aloft...hint--your calculations will involve the weight of the hailstone and the upward force of wind on the hailstone."

    and then a few questions later it asks: "hailstones typically reach terminal velocity before they hit the ground. Find the terminal velocity of the hailstones. hint--finding the terminal velocity of the stones is easy if you've completed the 1st question"

    is it possible that these questions are asking for the same thing?

    :uhh:

    thanks!
     
  12. Sep 27, 2004 #11

    NateTG

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    Perhaps the two are related. Does a hailstrone at terminal velocity have net acceleration? Is there a net force acting on it? What about a hailstone that is held aloft?
     
  13. Sep 27, 2004 #12

    i think i solved for terminal velocity with the equation that tide gave me.

    the first question should be solved by Fdrag = -1/2 CpAv^2 to get the air drag.

    right?
     
  14. Sep 27, 2004 #13

    NateTG

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    Yes, but you should be able to see similarities between the two problems.

    Perhaps you could consider the terminal velocity problem from the hailstone's reference frame?
     
  15. Sep 27, 2004 #14
    thank you!
     
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