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Homework Help: Frictional force on bend.

  1. May 24, 2010 #1

    fcb

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    1. The problem statement, all variables and given/known data
    A certain car experiances a limiting maximum frictional force equal to 75% of its weight. What is the smallest radius of bend that it can move around on a level road at 20ms-1


    2. Relevant equations

    F=mv2/r

    3. The attempt at a solution
    Couldnt get past last stage.
     
  2. jcsd
  3. May 24, 2010 #2

    Astronuc

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    Staff Emeritus
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  4. May 24, 2010 #3

    Doc Al

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    Staff: Mentor

    That's Newton's 2nd law applied to circular motion to give you the centripetal force. In this case, what is providing that force? (What is F equal to?) Plug in what you know about F and you'll be able to solve for v.
     
  5. May 24, 2010 #4

    fcb

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    Thanks for your fast and prompt reply.
    All i know about 'F' is that it is equal to 75% of the cars weight. I dont know the cars mass, How am i able to solve for F? I am lost in my own little world.
     
  6. May 24, 2010 #5

    Doc Al

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    Good!
    Call the car's mass 'm'. How would you express F in terms of m?
     
  7. May 24, 2010 #6

    fcb

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    Would it be F=.75 x 'm'
     
  8. May 24, 2010 #7

    Doc Al

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    Almost. Given the mass, how do you calculate the weight?
     
  9. May 24, 2010 #8

    fcb

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    0.75=1x202/r

    0.75=400/r

    r=[tex]\sqrt{}533[/tex]

    =23.06
     
  10. May 24, 2010 #9

    fcb

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    multiply it by acceleration due to gravity which is 9.8ms-2
     
  11. May 24, 2010 #10

    Doc Al

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    Right. W = mg, where g = 9.8 m/s^2.

    So revise your expression for F and plug it into the centripetal force formula.
     
  12. May 24, 2010 #11

    fcb

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    Scrap post #8. Its screwed.
     
  13. May 24, 2010 #12

    fcb

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    7.35=400/r
    400/7.35 = 54.42

    = 54.42
     
  14. May 24, 2010 #13

    Doc Al

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    Good! That radius will have units of m.

    Here's how I'd write it:

    F = mv^2/r

    .75 mg = mv^2/r

    The mass cancels (so you don't need to know the mass after all):
    .75 g = v^2/r

    so: r = v^2/(.75 g)
     
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