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Air Resistance formula

  1. Apr 22, 2003 #1

    dav2008

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    Hm..well Im taking a Physics B course and we never covered the topic of air resistance..
    So I was a little curious, so could anyone please give a brief explanation of the basics..or maybe some formula..

    I was just wondering if it was really simple like the boyency force or something or really complicated and thats why they didnt teach it to us..
    Tx
     
  2. jcsd
  3. Apr 22, 2003 #2
    Air resistance is drag. The calculation for drag is

    D=(Cd*A*R*v^2)/2

    Cd = is the drag coefficient and it is obtainable from wind tunnel testing data based on the shape of the object, and the reynolds number.

    R is the air density, which can be calculated or taken from charts

    A is the projected area normal to the flow

    v is relative velocity
     
  4. Apr 22, 2003 #3

    enigma

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    The thing which makes it tricky is that the drag acts in the direction opposite the motion at each point in time.

    That means that you don't have a simple formula to plug and chug. You need to look at the differential equations and (for most if not all cases) do step derivations to find the path.

    If you're familiar with differential equations and interested, I'll go into a little more detail.
     
  5. Apr 22, 2003 #4

    dav2008

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    Thanks for the info!

    Yea, thats what i was looking for...Im not taking calc yet...Going to take that next year.

    I just wanted a general idea what the air resistance was all about
     
  6. Apr 22, 2003 #5

    russ_watters

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    Drag is pretty easy when you consider air to be incompressible, but above about 220 mph you must consider the compressibility and the calculations are more complicated.
     
  7. Aug 3, 2003 #6

    TeT

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    hi there, i'm new (you see ;) ) and would like to know more about movement under air resistance or how its called (drag-movement?)... i'm from germany and do understand english, but not perfect...
    i have already some information about drag, but would be pleased if you could give me more information, not only about drag, but also about movement under air resistance... to be more specific i do not really need much infos about "more than 220mph" (balistic or how its called) but would be nice to read also some of it... hope you understand me?!
     
  8. Aug 4, 2003 #7

    enigma

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    Hi TeT,

    Welcome to the forums.

    The topic of drag and air resistance is pretty large. Could you be a little more specific as to what you're trying to find out about?
     
  9. Aug 4, 2003 #8

    TeT

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    hmm more specific? i will try...
    the situation is as follows: i have the task (as far as i know jet) about "movement under air-resistance" (or drag-motion or so (in german called "bewegung mit luftwiderstand")) and what i've done till now is to collect all information i could find about air-resistance... but always when i want to know more i get into another "topic" like air-planes, or balistic (bullets)... and as we have holidays now here in germany i dont knot what exactly my teacher want me to study about, so i'm waiting for some emails to be answered... furthermore i will do some test in school for showing drag, and thats (i think) the point, because when i record the test with a camera and do a report about the tests with some pictures in it, i think there will be no more place in the guideline of about 15 pages with given font and size of it, to talk much about drag by/of air-planes or so... so i think i should stay near my tests i will do... the tests are ... hmm how do i explain: we have i kind of "hover-board" in school ("luftkissenbahn" google says "air cushion course") with a plate on it for showing the drag... i dont know how to explain, but first o would like to know how to do some tests about drag... i have seen some on the internet, but our school do have have that "thing" with tells you the "Cd" (the picture below show the "thing" i mean), but nevertheless i think we have enough stuff in school to do some tests, so you could help me if you have any idea of doing a test or tell me with parts i MUST incude in my written work... (like to start with some deffinition of drag...)
    hope you understand me, if not, tell me and i will try to explain in a different way... thx for the replys ;)

    edit: damn i cant see the attached pic? why?
     

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  10. Aug 4, 2003 #9

    TeT

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    1. question: what is the difference of drag and air-resistance? thougt its the same thing, but as enigma worte:

    >The topic of drag AND air resistance is pretty large...

    i'm not sure
     
  11. Aug 4, 2003 #10
    There are actually two terms for air resistance, the one above proportional to velocity squared and another proportional to just velocity. In most cases, one or the other dominates. The determining factor according to Fowles and Cassady (Analytical Mechanics) is related to the velocity and the size.

    As enigma hinted, the equations get so ugly they can only be solved numerically for all but the simplest cases.

    For spherical objects, the ratio of the quadratic term to the linear term is
    c1 / c2 = 2.8e3 * v * r where r is the radius.

    So at low speeds, the linear term dominates and at high speeds the quadratic one dominates. For small objects like pollen, the linear term dominates.
     
  12. Aug 4, 2003 #11

    TeT

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    yeah i already found out that the drag is proportional to velocity squared and the (hmm german: "leistung" = achievement ?) is proportional to velocity*e3... and as i know i should show the proportional to velocity squared, so the tests will need a certain speed...
    hmm so much new words in here, i think i have to learn first all the terms i need in english...

    ps. thanks for replys so far... seems to be a great board (where you can find quick help...)
     
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