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Static Friction

  1. Feb 4, 2009 #1

    TG3

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    1. The problem statement, all variables and given/known data
    A block of mass 1.4 kg rests on a cart of 3.4 kg. The static friction between them is (mew) =.79
    What is the maximum acceleration in m/s^2 the cart can undergo over a frictionless surface before the block begins to slide?

    2. Relevant equations
    F (subscript ap) = mew x g x (m1 + m2)


    3. The attempt at a solution
    F = .79 x 9.81 x 4.8
    F= 37.19952 (I rounded appropriately when entering answer.)
    Yet I'm told this is wrong.... what am I missing?
     
  2. jcsd
  3. Feb 4, 2009 #2
    did you put on unit?
     
  4. Feb 5, 2009 #3

    TG3

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    The computer automatically adds a unit to the answer, you just have to make sure you solved for that unit. So yes...
    Is there anything wrong with my work?
     
  5. Feb 5, 2009 #4

    PhanthomJay

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    You are asked to find an acceleartion, yet you are solving for a force, and that calculation of the force is in any case not correct, as you seem to be trying to calculate a friction force acting on the block/cart system, which does not exist because the surface upon which the cart lies in frictionless. So start again, this time by drawing a free body diagram of the block. What force acts on the block in the horizontal direction just before it starts to slide? Once you identify that force, solve for its acceleration. Would not the cart have to have that same acceleration if the block must not slide?
     
  6. Feb 6, 2009 #5

    TG3

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    You're right. I did it over again like you said, but I'm still not quite there.
    I re-calculate the force of kinetic friction to be 10.84986 (9.81 x 1.4 x .79) but the answer is still being rejected.
    Am I miscalculating something, or making another conceptual error?
     
  7. Feb 6, 2009 #6
    Try mew*gravity.
     
  8. Feb 6, 2009 #7

    TG3

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    Hey- thanks!
    That was the first question in a set, so without the answer to that question, I couldn't get the rest, but with it they were pretty easy.
     
  9. Feb 6, 2009 #8

    PhanthomJay

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    That's all very nice that you now have the answer, but unless you try to understand why that's the answer, it is meaningless.
     
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