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Homework Help: Three Blocks

  1. Sep 25, 2010 #1
    1. The problem statement, all variables and given/known data

    Blocks of mass 5, 10, and 25 kg are lined up from left to right in that order on a frictionless surface so each block is touching the next one. A rightward-pointing force of magnitude 14 N is applied to the left-most block.

    a) What is the magnitude of the force that the middle block exerts on the rightmost one?
    Fright, middle = 8.749999995
    b) What is the magnitude of the force that the leftmost block exerts on the middle one?
    Fmiddle, left = ?
    c) Suppose now that the left-right order of the blocks is reversed. Now find the magnitude of the force that the leftmost block exerts on the middle one?
    F'middle, left = ?


    i figured out the first part i just need help to do the last two
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 25, 2010 #2
    I don't think (a) is right. Here's what I would do.

    F = ma

    The force pushing on the blocks is equal to their total mass times their acceleration but we have to solve for acceleration.

    14 = 30a

    a = 7/15

    Use F = ma for every question and don't forget Newton's 3rd law
     
  4. Sep 25, 2010 #3
    part a is correct because the homework is on a software and it accepted my answer
     
  5. Sep 25, 2010 #4
    how did you get a
     
  6. Sep 25, 2010 #5
    Nevermind. I screwed up. The total mass of the blocks is equal to 40 not 30. So a is right.
     
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