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Homework Help: Contact Forces

  1. Jul 11, 2007 #1
    Three blocks are in contact with eachother on a frictionless horizontal surface with

    A horizontal force of 24N is applied to M1.

    a) Find the acceleration of the three blocks

    b) Find the net force on each block

    c) Find the magnitude of the contact forces between the blocks

    I have successfully answered questions a) and b) I am confused about c)

    a) F = ma
    24N = (m1 + m2 + m3)a
    a = 4m/s^2

    b)Fm1 = 4N
    Fm2 = 8N
    Fm3 = 12N

    c) Thanks for any help with this part
  2. jcsd
  3. Jul 11, 2007 #2
    My main confusion lies with the fact that there are 3 blocks in contact with eachother and not just 2.

    Given the fact that Block one has a Fnet of 4N and a force being applied of 24N, a total force in the other direction must be 20N. But that would be from (m2 and m3) how would we get the force just from M2 on M1
  4. Jul 12, 2007 #3


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    The 20N force you have calculated is the normal contact force between the first and 2nd block, in satisfaction of Newton's 2nd Law. It is not correct to say that this force comes from either or both masses. Like supposing a 100N block sits at rest on a table. The normal contact force of the table on the block, from Newton's 1st law, is 100N upward, regardless of the table's mass.
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