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Finding friction force *without* mass or coefficient

  1. Oct 6, 2015 #1
    1. The problem statement, all variables and given/known data
    A car moving at 50mph coasts to a stop while travelling a distance of 150ft on level ground. what is the size of the friction force needed to bring this car to rest?

    2. Relevant equations
    vf^2 = vo^2 + 2a(xf-xo)
    F=ma
    Fk=un


    3. The attempt at a solution
    I first converted everything to meters and seconds.
    So, initial velocity is 22.35 m/s and distance travelled is 47.7 meters.
    Then, I found acceleration, which is -5.24 m/ss.
    Then, I found time. 4.27 seconds

    I tired to find the coefficient of friction with:
    F=ma=u(mg) (n is the same as mg because the surface is level)
    ma=u(mg)
    a=ug
    -5.24=u(-9.8)
    u=0.53

    I have no idea what to do now. It wants the friction force, not the coefficient.
    those are pretty much the only formulas we have at this point. We haven't gotten to work or energy formulas.
     
  2. jcsd
  3. Oct 6, 2015 #2

    berkeman

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    Staff: Mentor

    I agree that the force needed will depend on the mass of the car. Are you sure that is the whole problem statement? Can you post a (clear) picture or screenshot of the problem statement that you were given?
     
  4. Oct 6, 2015 #3
    I tried, but can't do it. I have photographs of it and my work saved on my computer but I can't upload them on here. I don't really have any place on the internet where I keep pictures either

    I looked at it again though, and it definitely says that :frown:
     
  5. Oct 6, 2015 #4

    berkeman

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    Staff: Mentor

    I'll PM you my e-mail address, in case you can e-mail them to me and I can post them for you.
     
  6. Oct 6, 2015 #5
    Maybe all they want is the force in g's.

    Chet
     
  7. Oct 6, 2015 #6

    berkeman

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    Staff: Mentor

    Yeah, that's kind of what I'm guessing now. There certainly is no mention of the car's mass in the problem statement, so I guess it just needs to show up in the answer as Amy did...

    PF Amy Problem.jpg

    PF Amy Work.jpg
     
  8. Oct 6, 2015 #7

    berkeman

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    Staff: Mentor

    BTW, Amy -- your files were JPG files, so you should be able to click the UPLOAD button in the Reply window to upload a *lJPG file as an attachment.

    Also, we usually do not want your work to be an image, and prefer that you type it into the PF directly. Just a note for future posts... :smile:
     
  9. Oct 6, 2015 #8

    robphy

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    Science Advisor
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    Gold Member

    Your answer could be expressed as "some specific fraction of the magnitude of the car's weight."
     
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