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Two blocks held by a rope on an incline

  1. Sep 15, 2012 #1
    1. The problem statement, all variables and given/known data

    A block with mass m1 = 9.4 kg is on an incline with an angle θ = 31.0° with respect to the horizontal.

    Now a new block is attached to the first block. The new block is made of a different material and has a coefficient of static friction μ = 0.9. What minimum mass is needed to keep the system from accelerating?

    2. Relevant equations

    Ft,1 = Ft,2
    Fg,1 = m1*g
    Fg,2 = m2*g
    Fn,1= Fg,1*cos 31
    Fn,2 = Fg,2*cos 31
    Ff,s,2 = Fn*.9


    3. The attempt at a solution

    Ft,1 = Ft,2
    Fg,1 = m1*g = 92.12N
    Fg,2 = m2*g = m2*9.8
    Fn,1= Fg,1*cos 31
    Fn,2 = Fg,2*cos 31
    Ff,s,2 = Fn*.9

    I'm not really sure what to do after this. I have some free-body diagrams set up, but since I can't post them here, I can't show them.

    I had a value for the coefficient of static friction, but that was for a system with a spring (.292), so I don't think that'll work here. I also have one for kinetic friction (.212), but that might not apply here. After that, I'm stuck.
     
  2. jcsd
  3. Sep 15, 2012 #2

    PhanthomJay

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    Please clarify this question, you are missing information.
     
  4. Sep 16, 2012 #3

    CWatters

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    Is the first block frictionless?
     
  5. Sep 16, 2012 #4
    The coefficients of friction are for the first block.
     
  6. Sep 16, 2012 #5

    PhanthomJay

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    Where's the rope? Please state the problem as written and provide a sketch if possible.
     
  7. Sep 17, 2012 #6

    CWatters

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    I'm sure this is a straightforward question but the way you have explained it is confusing...

    For example..

    is inconsistent with...

     
  8. Sep 17, 2012 #7

    CWatters

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    If we take your original post at face value....

    The first block is frictionless so it's simple to calculate the force down the slope due to gravity.

    The second block has friction so you can write two equations, 1) for the force down the slope due to gravity and 2) for the friction force acting up the slope.

    If the combination is static then the sum of all three forces equate to zero. Solve for the mass of the second block.
     
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