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Gauge pressure

  1. Nov 18, 2009 #1
    1. The problem statement, all variables and given/known data
    The gauge pressure in each of the four tires of an automobile is 240 kPa. If each tire has a "footprint" of 220 cm2, estimate the mass of the car.


    2. Relevant equations

    F= ma ?
    W1 = P1A1 ?
    3. The attempt at a solution

    total weight = W1+W2+W3+W4 = P(A1+A2+A3+A4) <------- since all tires have same surface area and same pressure.

    240pka(2.2m^3 x 4) = 2112 kg??

    dont think i did that right
     
  2. jcsd
  3. Nov 18, 2009 #2

    kuruman

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    You are confusing gauge pressure and absolute pressure. Look up the definition of gauge pressure.
     
  4. Nov 18, 2009 #3

    tiny-tim

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    Welcome to PF!

    Hi A_Munk3y! Welcome to PF! :smile:

    (try using the X2 tag just above the Reply box :wink:)
    erm :redface: … 220 cm2 = 220 m2/? :smile:
     
  5. Nov 18, 2009 #4
    Re: Welcome to PF!

    P = F/A ---- is that it?


    its 220 cm2 = 0.022m2 isnt it :redface:

    ok let me try this again :P
     
    Last edited: Nov 18, 2009
  6. Nov 18, 2009 #5
    attempt 2
    Pressure = Force/Area = Mg/Area = Pa
    kPa= = 1000 pa

    240Kpa x 1000 pa/kpa = 240,000 pa


    240,000 pa x 4 tires = 9.8 x m/ 880cm2(1/100)2

    960,000 = 9.8m/0.088m2

    or does the pressure remain 240000 (you dont add it?)
    so its 240,000 = 9.8m/.088m2
    m= 2155.1 kg?
     
    Last edited: Nov 19, 2009
  7. Nov 19, 2009 #6

    tiny-tim

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    Hi A_Munk3y! :smile:

    (just got up :zzz: …)
    hmm … you're really not good at units, are you? :wink:

    how about kN instead of kg? :smile:
    (that slipped past me last night! :rolleyes:)
     
  8. Nov 19, 2009 #7
    nope, i completely suck at units!!! >:(

    so you mean 240,000 = 9.8m/.088m2 is still not right ? :frown:
     
  9. Nov 19, 2009 #8

    tiny-tim

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    ah, you did put the 9.8 in in post #5 …

    yes, m = 240 x 88 / 9.8 = 2155 kg looks right (and I think 2000 kg is a fairly average car weight in the USA). :smile:
     
  10. Nov 19, 2009 #9
    ok! w000t!
    thank you so much :)
     
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