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Homework Help: Simple forces problem

  1. Mar 25, 2005 #1
    ok, our class got an assignment where we have to derive an equation for coefficient of static friction between our text book (inclined at an angle theta) and a coin. The vale of the angle and the mass of the coin are not given. I have come up with the following equation and need to know if it is correct.

    u = Ff(static)/Fnormal
    *using components of the force of gravity, i determined Ff to be FgxSINxTheta and Fnormal to be FgxCOSxtheta
    u = FgxSINxTheta/FgxCOSxtheta

    does this seem correct?

    thanks for looking.
  2. jcsd
  3. Mar 25, 2005 #2


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    Science Advisor
    Gold Member

    [tex]mg sin\theta[/tex] is the gravitational force down the ramp, so
    [tex]-(mg) sin\theta[/tex] is the force of static friction.

    [tex]mg cos\theta[/tex] is indeed the force normal


    [tex]\mu[/tex] must be positive, so
    [tex]\mu=\mid \frac{-(mg) sin\theta}{(mg) cos\theta}\mid[/tex]

    Which looks exactly like what you got except you call it Fg and I called it mg
  4. Mar 25, 2005 #3
    thanks, i just asked a friend and he got the same thing, so this must me right :smile:
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