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Homework Help: Need help on friction problem

  1. Sep 25, 2007 #1
    1. The problem statement, all variables and given/known data
    The drawing shows a large cube (m=25kg) being accelerated across a horizontal frictionless surface by a horizontal force P. A small cube (m=4kg) is in contact with the fron surface of the large cube and will slide downward unless P is sufficiently large. The coefficient of static friction between the cubes is 0.71. What is the smallest magnitude that P can have in order to keep the small cube from sliding downwards


    I know that the weight of the small cube is 39.2N. I also know that Fn is in the same direction as P. Past that, I'm confused. I have a equation: F=u*Fn, which is friction is equal to coef. of friction times normal force.

    Can someone explain to me what to do next?
     
  2. jcsd
  3. Sep 26, 2007 #2
    11 views, 12 hours and no help... guess you guys can help me or even tell me what else I need to put down...
     
  4. Sep 26, 2007 #3
    Hints

    On the small block, mass m, the downward force will be due to gravity mg.
    The upward force will be due to friction and a normal force from the acceleration uma.
    In order for the block not to fall uma=mg where u = 0.71
    Once you have solved for a, ask yourself how much force it takes to give that much a to the whole system.
    Tom
     
  5. Sep 26, 2007 #4
    Ok. I have W of 4kg = 39.2N. In order for the block to not fall, friction has to be 39.2N.

    mg=uma
    4*9.8=.71*ma
    39.2=.71*ma
    ma=Fn=55.2N.

    Where do I go from there? Can you at least tell me what m is supposed to be: 4kg or 25kg object?
     
  6. Sep 27, 2007 #5
    How much mass are you pushing?
     
  7. Sep 27, 2007 #6
    I am pushing 4kg to the right. Is that correct?
     
  8. Sep 27, 2007 #7
    we're not here to please you, be patient
     
  9. Sep 27, 2007 #8
    With what are you pushing the 4kg?
     
  10. Sep 27, 2007 #9
    You guys have been great. I've gotten help from my teacher and some fellow classmates. Guess this place just isn't good for those who seek help.
     
  11. Sep 27, 2007 #10
    Personally, I already know how to do the problem. I ask questions to try to guide you. Do you now understand the problem?
     
  12. Sep 27, 2007 #11
    friction of 4kg = weight of 4kg
    friction = uFn
    Ff=mg=uma
    mg=uma
    g=ua
    9.8=.71a
    a=13.8

    P=ma=(mass of big + mass of small)*13.8=400.2N
     
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