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Friction problem, need help getting started

  1. Oct 3, 2008 #1
    1.Here is the diagram of the problem.
    [​IMG]

    Two blocks attached by a string (see figure) slide down a 16° incline. Block 1 has mass m1 = 0.79 kg and block 2 has mass m2 = 0.27 kg. In addition, the kinetic coefficients of friction between the blocks and the incline are 0.30 for block 1 and 0.20 for block 2.
    (a) Find the magnitude of the acceleration of the blocks.

    (b) Find the tension in the string.

    Equation:
    Fk = Uk + FN
    F = MA

    (for individual mass)
    Fn = mg cos theta
    Fg = mg
    Fparallel = mg sin theta



    My trouble is how to start this. If this was just 1 mass, I would have no trouble but I get thrown off/confuse once they add a string and another mass.

    Can someone suggest on how I start this?
    I was thinking that M1 has the same Acc. as M2 but then I saw that they each have different kinetic coefficients. Can someone suggest on how to start this problem?
     
  2. jcsd
  3. Oct 3, 2008 #2

    Hootenanny

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    I good place to start would be to write down the sum of the forces acting on each block and then apply Newton's II law to each block individually.
     
  4. Oct 3, 2008 #3
    I drew a free body diagrams for each block.

    each mass has the same forces acting on them which is
    Normal Force
    Force of gravity
    Force of friction
    Tension Force

    This is what Ive been doing.
    For M1 I have as the sum of the forces
    (Force parallel)M1*g sin Theta + Tension Force - Friction Force = M1 * A

    For M2 I have as the sum of the forces
    (Force parallel)M2*g sin Theta - Tension Force - Friction Force = M2 * A

    Is this correct? If so can I add these 2 equations to cancel out the tension?
     
  5. Oct 3, 2008 #4

    Hootenanny

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    Looks good to me :approve:
     
  6. Oct 3, 2008 #5
    Awsome!! THanks!

    How would I calculate the tension?

    I found out the Acc = .115 m/s^2
     
  7. Oct 3, 2008 #6

    Hootenanny

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    You can either eliminate a from your two equations and then solve for T, or simply plug the value of a into either one of the equations and solve for T.

    It's always a good idea to check your answer by substituting both your values (T and a) into one or both of the equations to check that they work.
     
  8. Oct 3, 2008 #7
    Thanks so much for your help =].
    btw I like watching House too =]
     
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