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Finding the Coefficient of friction

  1. Mar 27, 2009 #1
    1. The problem statement, all variables and given/known data

    A 135 kg sled is being pulled along a horizontal surface at a uniform speed by means of a rope making an angle of 19.3 degrees with the horiontal. If the tension in the rope is 119 N Find the Coefficient of friction.

    2. Relevant equations

    u=Ff/Fn

    3. The attempt at a solution
    I have never done a question like this before but i fugured i probably had to find the force from the horizontal..im not sure if i did it right but i used trig to find that force and i got 112.3 N i then put that in the formula U=112.3/135 and i got 0.8319 but I really have no idea if i did it right.
     
  2. jcsd
  3. Mar 27, 2009 #2

    djeitnstine

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    You are correct. [tex]\mu F=F_{f}[/tex] so the ratio of the 2 must be the coefficient of friction. I also checked and our numbers match.
     
  4. Mar 27, 2009 #3
    Awsome...Thanks
     
  5. Mar 27, 2009 #4

    PhanthomJay

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    But don't forget that the weight of the block is not 135 N; its mass is 135 Kg. Also, the normal force fron the weight is offset by the upward component of the pulling force.
     
  6. Mar 28, 2009 #5
    HI i dont really get how u get u=112.3/135
    do you use the component method?
    here are my calculations...please tell me if they are right
    y components
    Fnet=Tensiony +Fnormal - Forcegravity
    0 =119*sin19.3 + Fnormal -135*9.8
    Fnormal= 135*9.8 -119*sin19.3
    =1362.33N
    Ff=1362.33*u
    x components
    Fnet= Fappx-Ff
    135*a=119cos19.3 - 1362.33
    0=119Cos19.3 - 1362.33 (because constant velocity, so acceleration= 0)
    U=119Cos19.3/1362.33
    =0.0824

    I am in grade 12 and I understand tht grade 12 and grade 11 methods are different, but please tell me what i did wrong and why i am not getting the right answer. THank you
     
  7. Mar 28, 2009 #6
    You are on the right track i had my teacher explain it to me and the answer is actually 0.0874
     
  8. Mar 28, 2009 #7

    PhanthomJay

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    see note above on math error; otherwise your method is very good.
     
  9. Mar 29, 2009 #8
    thanks guys...appreciate your help
     
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