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Newton's forces problem

  1. May 4, 2014 #1
    1. The problem statement, all variables and given/known data
    ScreenShot2014-05-04at33239PM_zps8bf4c472.png

    2. Relevant equations



    3. The attempt at a solution
    A)
    I am not sure how to go about solving this. I would say no you can't find Us. Can you just say that you do not know when the object will move?
    B) First I found acceleration x= 1/2 at^2, 18= .5a(9), a= 4.
    Then I used the free body diagram of m1. Fk pointing to the left, Tension 2 pointing to the right.
    T2- Fk= ma
    10g - 5g Uk = (5)(4), but my teacher uses the mass of both the blocks, 5+ 10. I am not sure why...
     
    Last edited: May 4, 2014
  2. jcsd
  3. May 4, 2014 #2
    I don't understand how there can be static AND kinetic friction between M1 and the surface..
     
  4. May 4, 2014 #3
    Not at the same time, obviously...
     
  5. May 4, 2014 #4
    Your mistake is that T2 is not 10g. Just do a force balance on M2 and you will see what it really is, and you will also find out why your teacher used (10 + 5).

    Chet
     
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