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Homework Help: Acceleration due to air resistance?

  1. Feb 23, 2007 #1
    a friend of mine asked if i could help him work out some equations for a (fairly) long range projectile, to go no farther than 2 miles or so. i have everything reasonably worked out except for acceleration due to air resistance. i'm using the equation from https://www.physicsforums.com/showthread.php?t=9066" thread to calculade Fd, and Cd=.295, rho=1.22752kg/m^3, A=pi.2^2m^2, v=240m/s, firing an object with a mass of .23kg. (Cd and rho both came from the Nasa website) I did the calculations and got an estimated acceleration of about (-)5698 m/s^2, which obviously cannot be right.

    I was also wondering if there is any way to get a more accurate velocity by factoring in acceleration of gravity and air resistance. I'd assume it'll take some nasty algebra/calculus, but i can't figure it out on my own.

    any help is greatly appreciated.
    Last edited by a moderator: Apr 22, 2017
  2. jcsd
  3. Feb 24, 2007 #2


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    Area = 4pi square meters and a mass of only 0.23 kg? That sounds pretty much like a 2 meter diameter balloon to me.

    If you tried moving something as big and light as that at 240 m/s (mach 0.7), the deceleration would indeed be large.
    Last edited: Feb 24, 2007
  4. Feb 24, 2007 #3


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    I think the area is 0.04pi m^2. Still very large. A projectile with a 20 cm diameter with a mass of only 0.23 kg canĀ“t fly at 240 m/s.
  5. Feb 24, 2007 #4
    Its not a wieldy problem, first Cd is not a constant at all but varies with velocity in a complex fashion, making this impossible to find an exact closed form solution. However, you could approximate a soln using a fixed Cd and initial V thats only subject to drag and gravity. But check the diameter, this is as pointed out much too low a ballistic coefficient to go anywhere due to whiffle ball effect.
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