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Homework Help: Pushing blocks

  1. Feb 6, 2010 #1
    1. The problem statement, all variables and given/known data

    http://www.public.asu.edu/~gbadams/f06/F424.gif

    Block1 = 2kg
    Block2 = 3kg
    Block3 = 4kg

    What is the force on block 1 by block 2

    2. Relevant equations

    F=ma

    3. The attempt at a solution

    i know this is easy but i cant figure it out.

    i found the acceleration of the 3 blocks

    8=9a
    a=0.889

    i know the net force on block 1 is 0.889(2kg) = 1.78

    I know the answer is 6.22N but i am not sure how to find it.
    any help would be great.

    thank you
     
  2. jcsd
  3. Feb 6, 2010 #2

    ehild

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    "i know the net force on block 1 is 0.889(2kg) = 1.78"

    Very good. You know that a force of 8 N acts to the first block. So what other force should be present to make the net force 1.78 N?

    ehild
     
  4. Feb 6, 2010 #3
    a 6.22N force should be present to add to the 1.78N net force to equal the 8N force in the opposite direction so they cancel out to zero correct?
     
  5. Feb 6, 2010 #4

    ehild

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    No. 8 N - 6.22 N = 1.78 N causes 0.889 m/s^2 acceleration of the first block. Where comes this 6.22 N come from?

    ehild
     
  6. Feb 6, 2010 #5
    "where comes this 6.22 N come from?"

    i do not quite get what you are asking here?

    i understand that Fnet = Force applied + (force of block 2 on block 1)



    1.78N = 8N + x

    solve for x to be -6.22N or just 6.22N to the left and that is the force on block 1 by block 2
     
    Last edited: Feb 6, 2010
  7. Feb 6, 2010 #6

    ehild

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    OK. You solved the problem.
     
  8. Feb 6, 2010 #7
    Thank you for your help and explanations :)
     
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