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Homework Help: Terminal Speed

  1. Oct 2, 2004 #1
    What is the terminal speed of a 6.5 kg spherical ball with a radius of 3.0 cm and a drag coefficient of 1.50? The density of the air through which the ball falls is 1.2 kg/m3.

    I used vt=sqrt((2Fg)/(CpA)). Where Fg is force of gravity, C is drag coeff., p is air density , and A is cross sectional area. When I plug in the numbers I get the wrong answer. I used 1.5 as C, 1.2 as p, 9.8 as Fg, and (9*pi) as a (area of circle-pi*r^2). What am I doing wrong.
  2. jcsd
  3. Oct 2, 2004 #2
    I think you need to convert your radius to meters.
  4. Oct 2, 2004 #3
    I forgot to convert lol but that doesn't help, answer is still wrong.
  5. Oct 2, 2004 #4
    Never Mind Figured it Out :)
  6. Oct 2, 2004 #5


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    Science Advisor
    Gold Member

    Your expression hasn't got units of velocity.

    I think you have forgotten to include the sphere mass in Fg.
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