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

  1. Jan 8, 2015 #1
    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= (5m)a
    a= F/(5m)

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

    Attached Files:

  2. jcsd
  3. Jan 8, 2015 #2
    What force is necessary to produce your calculated acceleration in the second block?
  4. Jan 8, 2015 #3
  5. Jan 8, 2015 #4
    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.
  6. Jan 9, 2015 #5
    I see. Why not use the mass 3m?
  7. Jan 9, 2015 #6


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    Either will do provided you bring all relevant forces acting on the mass into the equation.
  8. Jan 9, 2015 #7
    but if i were to use f=3m, and a is F/5m, i wouldnt get 2f/5 as the answer
  9. Jan 9, 2015 #8


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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.
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