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Homework Help: Working out Coefficient of Friction HELP!

  1. May 29, 2008 #1
    1. The problem statement, all variables and given/known data

    For my Year 12 Physics EEI (Yes I have already asked for help about the actual prac, but now I'm onto writing up the theory:wink:) I conducted a prac where I rolled a fishing sinker down a 1.2m length of pipe into a tray of flour. I am now trying to use all relevant theory for the write up. I have been told (by classmates not the teacher) that as part of my theory I need to include the force of friction equation.

    2. Relevant equations

    I can work out the Normal force, by using the eqn Fn=mg, but without knowing Fk (or Fs, but I said Fk because the sinker is rolling down the pipe, not stationary), or u (coefficient of friction, sorry I didn't use the correct symbol) how can I work out u? How can I find it with only knowing one variable?

    3. The attempt at a solution

    I found the rearranged formula and it seemed simple enough: u =Fk/N, but as I said there are too many variables. I know the weight of the sinker, the lengths of the "ramp" (being the pipe) and the distance along the ground, and the angle.

    How is it that I use cosine in working out in this formula? Sorry I know this is a basic question but we haven't done this work in class for about year, still no excuse though. :frown:

    Any help is thoroughly appreciated :smile::biggrin:
  2. jcsd
  3. May 29, 2008 #2


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    The normal force is not just mg. It's the component of the vertical force mg that is normal to the pipe. If the angle of the pipe is 't' with respect to the horizontal, then that's mg*cos(t).
  4. May 29, 2008 #3
    So does mg*cos(72) = the Normal force? Or is it the force of friction? That gives 307.83. Is this Newtons? I'm assuming it is.

    Thank you for your help =) .
    I should be understanding this. Grr.
  5. May 29, 2008 #4


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    Normal force, if 72 degrees is the angle between the pipe and the horizontal. Didn't I say that? For example if the angle were 90 degrees, then the pipe would be vertical, so no normal force (since cos(90 degrees)=0).
  6. May 29, 2008 #5
    OK yep, sorry, yes I get that now.

    From there I can go Us = sin 72 degrees / cos 72 degrees = (which is the same as) tan 72 degrees = 3.077 (Static Coefficient).

    How different is the static coefficient from the kinetic, and how do I then work that out?

    Wait...because my angle is measured when the object was moving, not just starting to, does that mean that I have already worked out the kinetic and it is infact kinetic not static friction?
    Last edited: May 29, 2008
  7. May 30, 2008 #6


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    Yes, if it's moving it's kinetic.
  8. May 30, 2008 #7
    Thank you so much, sorry my question and answers were frustrating, thank you for continuing to help me.:shy::surprised:approve::biggrin:
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