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Caculating tractive force (i need a formula)

  1. May 11, 2007 #1
    i ve got the following question from my motor vechile diploma

    kerbside weight 1360kg
    rolling resistance 140N per 1000kg
    frontal area 2m(squared)
    drag coefficient 0.025

    caculate the total tractive resistance when the car travels at 70 km/h on a level road?

    what i need is the formula to work out tractive resistance ive searched for about 3 hours on the net but no luck.

    any help appreciated

    dan
     
  2. jcsd
  3. May 11, 2007 #2

    Hootenanny

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    Firstly, ask yourself what is tractive resistance?
     
  4. May 11, 2007 #3
    is it traction
     
  5. May 11, 2007 #4

    Hootenanny

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    Which is what in terms of the forces acting on a vechicle?
     
  6. May 11, 2007 #5
    drag force
     
  7. May 11, 2007 #6

    Hootenanny

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    Which would include the rolling resistance of the wheels and the fluid friction from the air, yes? So can you write down an equation for these two forces?
     
  8. May 11, 2007 #7
    i can work out the rolling resistance and air resistance which is
    rolling resistnce = 190.4N
    air resistance = 452.76N

    what i need is a formula so i can caculate the total tractive force
     
  9. May 11, 2007 #8

    Hootenanny

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    So, if the tractive force is the total or sum of all the drag forces acting on the vechicle, what do you think you should do now?
     
  10. May 11, 2007 #9
    add them together
     
  11. May 11, 2007 #10

    Hootenanny

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    Sounds good to me :approve:
     
  12. May 11, 2007 #11
    ok heres the full question

    kerbside weight 1360kg
    rolling resistance 140N per 1000kg
    frontal area 2m(squared)
    drag coefficient 0.025

    caculate the rolling resistance of the car?
    caculate the gradient resistance when the car climbs a gradient of 1/20
    caculate the air resistance of the car when it is travelling on a level road at 100 km/h?
    caculate the total tractive resistance when the car travels at 70 km/h on a level road?

    ive answered the top 3 questions but havent used drag coeeficcient
    and i thought that might come in to tractive resistance but obvisily not. so where would that come in to surly they couldent of just told you it for the sake of it.
     
  13. May 11, 2007 #12

    Hootenanny

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    You should have used the drag coefficient in calculating the air resistance of the car.
     
  14. May 11, 2007 #13
  15. May 11, 2007 #14

    Hootenanny

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    My best guess is that you are required to use Stoke's-type formula to calculate the drag;

    [tex]F = -bAv[/tex]

    where b is your drag coefficient. And we usually work in SI units, m, m/s2, N etc.
     
    Last edited: May 11, 2007
  16. May 11, 2007 #15
    im abit confused now , what does the v mean.
    so that formula on the website is wrong ?
     
  17. May 11, 2007 #16

    Hootenanny

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    v is the velocity or speed. No, the formula on that website is useful in some cases, but here you are given your own drag coefficient so you should use that. This formula is essentially the same as the one given on the website, with the exception of your own drag coefficient. Note, the correction in my previous post, the addition of cross sectional area A.
     
  18. May 11, 2007 #17
    is air resistance the same as drag resistance or are they diffrent?
     
  19. May 11, 2007 #18

    Hootenanny

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    For your purposes they're exactly the same.
     
  20. May 11, 2007 #19
    so 0.025 x 19.444(m/s) = 0.4861 for stokes formula

    0.0462 x 70^2(km/h) x 2 = 452.76N for the websites formula

    these are 2 diffrent answers

    thats where im confused
     
    Last edited: May 11, 2007
  21. May 11, 2007 #20

    Hootenanny

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    I really don't know where your getting your numbers from, don't forget the area in Stokes formula and that the website uses different units. What are the units of your drag coefficient.
     
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