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Homework Help: Contact Forces Question(s) part 2

  1. Oct 6, 2008 #1
    1. The problem statement, all variables and given/known data
    Three blocks are placed onto a frictionless horizontal surface. These blocks are side by side and are in contact with each other. Let m1=2m2=5m3. Answer the following exercises.
    (Part 1)
    a)If a force is applied to m1, then show that

    i)Fa = 17/2m3a

    ii)F12 = 7/2m3a where F12 is the contact force of mass 1 on mass 2

    iii)F23=m3a

    Question: Consider the same three blocks as in the question above. If these blocks rest upon a rough floor (μ=0.11), show that if a force is applied to m1, then:
    (Part 2)
    i)Fa = 17/2 m3(a+μg)

    ii)F12 = 7/2 m3(a+μg)

    iii)F23 = m3(a+μg)


    2. Relevant equations
    Σ Fx = ma


    3. The attempt at a solution
    All in all both questions (part 1 and part 2) are more or less the same except this question (part 2) includes the force of friction while the other (part 1) does not. I have already solved all the questions for part 1, and where I am having difficulty is, is in getting rid of the mass (m) in "μmg" for this question so that I could get the values as seen above.
     
  2. jcsd
  3. Oct 6, 2008 #2
    Never mind I figured out where I went wrong and now know what to do for this question so a reply is not needed. But thank you to those who were trying to help by trying to explain the question to me.
     
  4. Oct 6, 2008 #3
    It would be good to see your attempt at the questions, then maybe I could understand what you are saying a little better. From what I have read you are finding it difficult cancelling the masses in all of part 2.

    F = ma
    Therefore you have ma2 = 17/2 m3 (a+μg)
    Divide by m.
     
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