Friction between two surfaces

  1. May 13, 2007 #1
    1. The problem statement, all variables and given/known data

    hi, i am almost done my lab on the coefficient of friction on an inclined plane. I was attempting to prove that the coefficent of static friction would be the same no matter the weight of the mass that was static on the incline. However, i plotted a coefficient versus mass graph, and the line wasn't horizontal (the coefficient changed). My quick question was whether i still need to find the equation for finding the coefficient based on my results, or if i could just conclude that in my case, the coefficient didn't stay constant, and then move to my conclusion

    2. Relevant equations



    3. The attempt at a solution
     
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  3. May 13, 2007 #2

    Hootenanny

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    How did you calculate the coefficient of friction for your results?
     
  4. May 13, 2007 #3
    i had an incline, measured the weight of the mass, and changed the angle such that if i increased the angle by a little bit, the mass would slide. I did this for several different masses and recorded the angle. I then used a freebody diagram and the equation for static friciton (u = Fmax/R)
     
  5. May 13, 2007 #4

    Hootenanny

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    And how did you calculate the normal reaction force?
     
  6. May 13, 2007 #5
    R = mg cosx
    F = mg sinx

    so in essence, the coefficient was tanx
     
  7. May 13, 2007 #6
    technically, all i need to know is whether i would have to process the graph to find a relation between weight and the coefficient (although i know that technicallly the coefficient is independant of mass) if all i wanted to prove in the lab was that the coefficient was independant of mass (and in my case, my experimental data didn't prove it)
     
  8. May 13, 2007 #7

    Hootenanny

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    Your not actually plotting [itex]\mu[/itex] vs. [itex]m[/itex] what your actually plotting is [itex]\tan\theta[/itex] vs. m since;

    [tex]\mu = \frac{F}{R} = \frac{mg\sin\theta}{mg\cos\theta} = \tan\theta[/tex]
     
    Last edited: May 13, 2007
  9. May 13, 2007 #8
    but since tanx = u, so why should it matter whether you plot u vs. m or tanx vs m?
     
  10. May 13, 2007 #9

    Hootenanny

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    Indeed, [itex]\tan\theta = \mu[/itex], but are you keeping the angle constant?
     
  11. May 13, 2007 #10
    no, because the greater mass wouldn't stay static at the certain angle, so it would decrease.

    if i am trying to prove that the coefficient should stay the same, and it doesn't in my experiment, do i have to do more, or can i just stop once the graph shows that the coefficient changed?
     
  12. May 13, 2007 #11
    i thought that: Frict max=mu R
    so for your experiments to prove mu is irrespective of mass the particle has to be on the point of slipping
    that may be too simple i am only doing as physics and you guys might be talking about something higher tho from mechanics mu is >= to R/Frict max
    (sorry about symbols ect)
     
  13. May 13, 2007 #12
    honestly, the only question i wanted answered was in respect to the set up to the lab. Once you process enough data to prove/disprove the purpose of the lab, do you have to do any more processing, or can you just go straight to conclusion/evaluation?
     
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