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Tough question about reaction force Newton's third law -- Please help

  1. Jan 8, 2015 #1

    bomberkid

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    1. The problem statement, all variables and given/known data
    . question is as attached. answer is 2f/5..how do u get this?
    2. Relevant equations
    F=ma
    F= (5m)a
    a= F/(5m)

    3. The attempt at a solution
    gotten a=F/5m , but why the answer is 2f/5?
     

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    bomberkid, Jan 8, 2015
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  3. Jan 8, 2015 #2

    Joffan

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    What force is necessary to produce your calculated acceleration in the second block?
     
    Joffan, Jan 8, 2015
  4. Jan 8, 2015 #3

    bomberkid

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    F?
     
    bomberkid, Jan 8, 2015
  5. Jan 8, 2015 #4

    Joffan

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    No... you calculated the specific ##a##, now put that back into ##F=ma## along with the second block's mass

    Unfortunately the confusion between the abstract "F" and the specific "F" in this problem is perhaps making understanding a little harder than it should be.
     
    Joffan, Jan 8, 2015
  6. Jan 9, 2015 #5

    bomberkid

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    I see. Why not use the mass 3m?
     
    bomberkid, Jan 9, 2015
  7. Jan 9, 2015 #6

    haruspex

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    Either will do provided you bring all relevant forces acting on the mass into the equation.
     
    haruspex, Jan 9, 2015
  8. Jan 9, 2015 #7

    bomberkid

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    but if i were to use f=3m, and a is F/5m, i wouldnt get 2f/5 as the answer
     
    bomberkid, Jan 9, 2015
  9. Jan 9, 2015 #8

    haruspex

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    You get the right answer if you bring in all forces acting on the mass and do the algebra correctly. Please post your working.
     
    haruspex, Jan 9, 2015
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