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Homework Help: Newton's 3rd Law

  1. Oct 21, 2008 #1
    1. The problem statement, all variables and given/known data

    Three blocks are in contact with one another on a frictionless, horizontal surface as shown in Figure P4.43. A horizontal force vector F is applied to m1. Taking m1 = 2.00 kg, m2 = 3.00 kg, m3 = 5.00 kg, and F = 16.0 N, draw a separate free-body diagram for each block. (Do this on paper. Your instructor may ask you to turn in this work.)

    I already answered parts a and b, i just need help with c:
    (c) Find the magnitude of the contact forces between the blocks.
    m1 and m2 ________ N
    m2 and m3 ________ N

    2. Relevant equations

    i know that i need to use newton's third law of an object acting upon another object.
    F = ma

    3. The attempt at a solution

    i tried doing F(m2/m1 + m2) for the first one and got 12, however the answer was wrong and the online program i have to use to turn it in says i was within 10% of the correct answer.

    so now i'm stuck and don't know what to do.
  2. jcsd
  3. Oct 22, 2008 #2


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    Homework Helper

    Hi anteaters,

    Can you show how you got this expression? It looks rather close to being correct (it would be right if m3 was not there), so maybe if you post your work someone can point out what's going wrong.
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