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

  • Thread starter bomberkid
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  • #1
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Homework Statement


. question is as attached. answer is 2f/5..how do u get this?

Homework Equations


F=ma
F= (5m)a
a= F/(5m)

The Attempt at a Solution


gotten a=F/5m , but why the answer is 2f/5?
 

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Answers and Replies

  • #2
473
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What force is necessary to produce your calculated acceleration in the second block?
 
  • #3
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F?
 
  • #4
473
13
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.
 
  • #5
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I see. Why not use the mass 3m?
 
  • #6
haruspex
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I see. Why not use the mass 3m?
Either will do provided you bring all relevant forces acting on the mass into the equation.
 
  • #7
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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
 
  • #8
haruspex
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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
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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