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Homework Help: Centripetal accel

  1. Dec 16, 2003 #1
    Truck weighs 6000 lbs The truck is going around a curve with a radius of 650 ft. The max friction force the road can excert is 1125 lbs.Its velocity needs to be calculated ft/sec.

    Now I now F=ma and Ff = m*(v^2/r)
    I also now that 1 pound = 4.448 N

    problem is i have 3 different answers by trying it different ways

    a) v = 62.6 ft/sec
    b) v = 11.04 ft/sec
    c) v = 58.88 ft/sec

    Which one is the right one

    This is the final problem I have to do before i have to turn in my physics notebook and take my final exam today at 9 am

    any assistance would be great

    thanks
     
  2. jcsd
  3. Dec 16, 2003 #2
    B) is correct
    i dont know how u reached the other results Pls show it
     
  4. Dec 17, 2003 #3
    I think there is a mistake in dimension somewhere in there. Actually, the frictional force must be converted to Newtons and the weight of the truck must be converted to kg. Then, you can use [

    f = ma
    f = v^2 / r

    (1125 lbs)*(4.448 N/lbs) = [(6000 lbs * 4.448 N/lbs)/9.81 m/s^2]*v^2/r

    You will get around 34 ft/sec.
     
  5. Dec 17, 2003 #4
    B is correct harsh u are given max frictional force why u are dividing by 9.81
     
  6. Dec 17, 2003 #5

    HallsofIvy

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    Science Advisor

    Why? You are given all the information in the "English" system and asked for the result in the "English" system. F= ma and F= mv2/r are true in any system. Why convert to metric?
     
  7. Dec 17, 2003 #6
    Right, but then you dont really get the correct units.
    Its like you are canceling pounds over pounds, so the left side of the equation is unitless.
     
  8. Dec 17, 2003 #7
    It should be 1125lbs m/s^2
     
  9. Dec 19, 2003 #8

    ShawnD

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    Science Advisor

    I'm getting the first answer. I've never actually used the imperial system before; it's just weird.

    [tex]F = \frac {mv^2}{r}[/tex]

    [tex]\frac {Fr}{m} = v^2[/tex]

    we have to find the mass of the thing. divide the weight by the acceleration to get the mass.

    [tex]m = \frac {F}{a}[/tex]

    [tex]m = \frac {6000}{32.2}[/tex]

    [tex]m = 186.3[/tex] i think that's in slugs

    now back to the other formula

    [tex]v = \sqrt {\frac {Fr}{m}}[/tex]

    [tex]v = \sqrt {\frac {(1125)(650)}{186.3}}[/tex]

    [tex]v = 62.65[/tex]
     
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