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Homework Help: Friction problem?

  1. Oct 12, 2009 #1
    1. The problem statement, all variables and given/known data
    In the diagram shown below, the lower block is acted on by a force, F, which has a magnitude of 64.1 N. The coefficient of kinetic friction between the lower block and the surface is 0.346. The coefficient of kinetic friction between the lower block and the upper block is also 0.346. What is the acceleration of the lower block, if the mass of the lower block is 4.04 kg and the mass of the upper block is 1.86 kg?

    http://img266.imageshack.us/img266/7450/knpic0828.png [Broken]

    2. Relevant equations
    ΣF y = ma y

    ΣF x = ma x

    3. The attempt at a solution

    So i set up 2 freebody diagrams, one for each object and I made F net equations. For the top block:

    ma = T - μk*mg (1)

    The friction comes from the friction between the top block and the bottom block.

    and for the bottom it would be:

    ma = Fa - Fk - T (2)

    In this case the friction comes from the friction between the bottom block and the ground.

    So in the second equation I solved for T, subbed it into equation 1 and solved for a, however I am not getting the correct answer. Any ideas?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Oct 12, 2009 #2
    What happened to the friction between the two blocks in your second equation?
  4. Oct 12, 2009 #3
    How would I write the friction between the two blocks?

    ma = Fa - Fk - T - Fk (?)

    So would I get,

    ma = F - μk* m1g - μk * m2g - T
  5. Oct 12, 2009 #4
    Yes, I think that's correct.

    [tex]m_2 a= F-(m_1+m_2)g\mu_k-T[/tex]
  6. Oct 12, 2009 #5
    Hey I still cant get it with that formula.. Can someone help?
  7. Oct 13, 2009 #6
    Did you find the expression for [tex]a[/tex]?
    Last edited: Oct 13, 2009
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