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Homework Help: Contact Forces Question(s)

  1. Oct 5, 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.

    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


    2. Relevant equations

    3. The attempt at a solution
    Fx1=m1a Fx2=m2a Fx3=m3a
    Fa-F21-F31=m1a F12-F32=m2a F13+F23=m3a
    Fa=m1a+F21+F31 F12=m2a+F32 F13=m3a+F23

    I'm not quite sure if this is the way to even go about it. Any help with this would be greatly appreciated.
  2. jcsd
  3. Oct 5, 2008 #2


    User Avatar
    Homework Helper

    Welcome to PF.

    First of all observe that
    Total mass is 1.7m1.
    m2 = .5m1
    m3 = .2m1

    F = m*a = 1.7m1 = 1.7*(5m3)*a = 17/2*m3*a

    Figure the subsequent contact forces by looking at the forces acting on the subsequent blocks in isolation.
    Last edited: Oct 5, 2008
  4. Oct 6, 2008 #3

    After going through what you mentioned, I figured out how to do the question and subsequently the questions that were related to it. Thanks for the help.
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