Coefficients of friction

  1. Are coefficients of friction Ms and Mk independent of the weight being pulled?

    We just had a lab on friction and i am just curious about this.

    Thank you
     
  2. jcsd
  3. tiny-tim

    tiny-tim 26,054
    Science Advisor
    Homework Helper

    Hi mybrohshi5! :smile:

    (have a mu: µ :wink:)

    In examination questions, µs and µk are always independent of weight. :wink:

    In reality, see this, from the PF Library on friction:

    Tables of coefficients of friction:

    Many tables can be found on a web search, eg http://engineershandbook.com/Tables/frictioncoefficients.htm, which begins
    "Extreme care is needed in using friction coefficients, and additional independent references should be used. For any specific application the ideal method of determining the coefficient of friction is by trials. A short table is included above the main table to illustrate how the coefficient of friction is affected by surface films. When a metal surface is perfectly clean in a vacuum, the friction is much higher than the normal accepted value and seizure can easily occur."
    For some materials, the coefficient can be greater than one, and for solids on rubber it can be as high as four.

    Increasing pressure between dry surfaces may increase the coefficient, at first slightly, but eventually very quickly, leading to seizing. For this and other factors affecting coefficients of friction, see the top box in http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm
     
  4. Thanks tiny-tim.

    In my lab we used about a 1 kg weight on top of a wooden block and that was dragged across the table.

    So say i reduced that weight down to 0.5 kg and dragged it again would the coefficients of friction be the same or different?

    From what i got from the websites you posted i think the coefficients of friction will be independent of the weight therefore making them stay the same because....

    ....since relatively low pressures are being used then the friction would go down as the weight goes down and the friction would go up as the weight goes up (to an extent) so basically i am saying that the friction is directly proportional to the weight so µk and µs will stay the same making them independent of the weight being pulled.....
     
  5. tiny-tim

    tiny-tim 26,054
    Science Advisor
    Homework Helper

    Hi mybrohshi5! :smile:

    Should be the same, for exactly the reasons you've given.

    (But you should still try it!! :wink:)
     
  6. Thank you. That helped clear up a lot of things for me :)
     
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