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Homework Help: Coefficients of drag, Cd -

  1. Nov 19, 2005 #1
    What is the coefficient of drag, Cd, of a tennis ball (roughly d = 6,5 cm, m = 57 g)?

    Generally; where do I find such coefficients? I'd like to find coefficients for projectiles in water too - since these are supposedly not the same, because of the water viscosity. I've found the "Drag Coefficient" text on Scienceworld Wolfram, which apparently equals Cd to Re^(-1/2), where Re is the Reynolds number. But the equation from which the text gets Cd has L in it, the "size scale" of the body - which is squared. But in my equation there is no such scale, only the silhouette area A of the body, which isn't squared. Why this difference?

    Also - the "length scale" l is in the Cd equation. What is it?

    I'm quite puzzled. If the Cd can be found directly from a Reynolds number, which I'm not sure it can, could my problem then be solved?
  2. jcsd
  3. Nov 22, 2005 #2


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    Reading Wolfram, it specifically quotes beside the use of the size scale of the body L to refer to (Tritton 1988, p. 93). The formula for drag is quite approximatish. From the text I'm familiar with as also found here http://www.lerc.nasa.gov/WWW/K-12/airplane/dragco.html [Broken]

    the L is instead an area A. The choice of this reference area A can vary.

    In Wolfram, the Cd being directly a function of the Reynolds number is from successive approximation.
    Last edited by a moderator: May 2, 2017
  4. Nov 23, 2005 #3


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    Cd for something like a tennis ball is going to vary greatly due to surface conditions. There are standard Cd values for smooth objects like a sphere, but in your case, they would be introducing more error into your calcuations.

    From what I have seen, when there is a nice easy relationship like the one you stated for Re and Cd, that usually means there's a pretty healthy restriction on it's usage. For example, I have a source right here that says the Cd for a sphere is 24/Re. However, that is for Re<1. Probably not very helpful. The Cd is not going to be a nice straight line over a decent range of Re. I do have a chart for the Cd vs. Re of a smooth cylinder and sphere for Re going from 10^-1 to 10^7. I just don't have a way to post it. If you can provide an Re range, I can at least read off some values for you if you think that would help.
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