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Homework Help: Connected objects w/ friction

  1. Feb 4, 2009 #1
    1. The problem statement, all variables and given/known data
    A block of mass m1 = 2 kg and a block of mass m2 = 3 kg are tied together and are pulled from rest across the floor by a force of Fp = 20 N. The coefficient of friction of the blocks with the floor is µ = 0.23.



    2. Relevant equations
    What is the acceleration of the two blocks?


    3. The attempt at a solution
    a=F/(m1 + m2)
    a=20N/(2kg + 3kg)
    a=4 m/s^2 ???

    I know that somehow µ = 0.23 is involved, but i dont know where to use it.
     
  2. jcsd
  3. Feb 4, 2009 #2

    Delphi51

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    u = 0.23 is the coefficient of friction, used in the formula F = u*Fn,
    where Fn is the "normal" force pressing the object against the surface it is moving on.
    Begin by calculating the force of friction on each block.
     
  4. Feb 4, 2009 #3
    I still dont understand. What is the force of Fp=20 N? and where does that fit in
     
  5. Feb 4, 2009 #4

    Delphi51

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    Someone is pulling the blocks with a force of 20 N.
    There is a friction force opposing that, which you are about to calculate.
    Once you have the two forces acting on the blocks, you can use F = ma to find the acceleration.
     
  6. Feb 5, 2009 #5
    I tried calculating the force of friction on each block, but my answers still werent correct. what is the correct set up? thats all i need to know to see where i made the mistake.
     
  7. Feb 5, 2009 #6

    Delphi51

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    Use F = u*Fn to find the force of friction. Show your calc and we'll find your error.
     
  8. Feb 5, 2009 #7
    F1=u*Fn
    -mg=u*Fn
    -mg/u*=Fn
    -2*9.81/0.23=-85

    F2=-mg+Fp+u*Fn
    mg-Fp=u*Fn
    mg-Fp/u*=Fn
    3*9.81-20/0.23=41
     
  9. Feb 5, 2009 #8

    Delphi51

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    "-mg+Fp+u*Fn " is not correct.
    mg, the weight, is downward while Fp is horizontal.
    You can't add forces in different directions.

    First, figure out the force of friction.
    Ff = u*Fn = ???
    Fn is the normal force, pressing the blocks against the surface. It is caused by gravity. Can you work it out?

    After you get Ff, write "sum of forces = ma" for the horizontal direction.
    You'll have the 20 N pulling force and the friction force opposing the motion.
    Solve for acceleration.
     
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