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Question about drag force

  1. Jul 5, 2010 #1
    What is the drag force on a 1.6 m wide, 1.4 high car traveling at

    A) 10 m/s B) 30 m/s

    I'm getting 68 N for part a and 615 N for part B using equation D=.25pAv^2


    P= density of air (1.22 kg/m^3) and A is cross sectional area of obect as it faces the wind). *I plugged in 1.22 for p, 2.24 for A, and 30 for v but this is wrong according to the book. What am I doing wrong?

    And I don't think I need it to solve this problem but *v terminal = sq rt (4mg/pA).
     
  2. jcsd
  3. Jul 5, 2010 #2

    PhanthomJay

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    I don't know how you can solve this problem without knowing the drag coefficient (C_d)for the car. The formula you note is for certain shapes with C_d =0.5; a car could have a greater or lesser C_d depending on its design. (Note: The terminal velocity equation you have noted is for a falling object with a C_d =0.5; it doesn't apply to objects traveling on a level surface.)
     
  4. Jul 5, 2010 #3
    my book says to consider Cd as .5 so ive already included it in the equation. The formula is actually .5CdpAv^2 but my books says Cd is .5 so the equation becomes D=.25pAv^2 so is my answer right? thanks
     
  5. Jul 5, 2010 #4

    PhanthomJay

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    Looks OK to me. Maybe it's a significant figure thing.
     
  6. Jul 5, 2010 #5
    Thank you kindly for all your help!
     
  7. Feb 6, 2011 #6
    I'm stuck on the same problem about a car that is 1.6 meter wide and 1.4 meters tall and I have to find the drag force for it at 10 m/s and 30 m/s. But I got the wrong answer when I plugged it into the equation D= .25(rho)(Area)(Velocity^2). I found out that the velocity of Air is 1.29 kg/m^3 and plugged that in for rho but still no luck. Help!!!
     
  8. Feb 7, 2011 #7

    PhanthomJay

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    Assuming quadratic drag and that Cd/2 = 0.25,, the equation is correct, where rho is the mass density of air. I don't know what the book assumed or gave for the drag coefficient.....a mack truck would have a much greater drag cofficient than one of those modern 'bird shaped' vehicles.
     
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