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Kinetic/ Static friction- Newtons 3rd Law

  1. Sep 30, 2009 #1
    1. The problem statement, all variables and given/known data

    The coefficient of static friction is 0.60 between the two blocks in figure. The coefficient of kinetic friction between the lower block and the floor is 0.20. Force causes both blocks to cross a distance of 5.0 m, starting from rest.
    What is the least amount of time in which this motion can be completed without the top block sliding on the lower block?

    2. Relevant equations

    3. The attempt at a solution

    Fnet Box 1: T-(Fn x 0.6)=ma

    Fnet Box 2: T- (Fn x o.2)=ma

    Where does static friction play into Fnet of box 2? I almost got it but the only way I could get the answer would be to have static friction in the positve direction. So it would be something like: Fs-Fk=ma . But I dont get how the static friction is in positive direction. please Help!!!!!
  2. jcsd
  3. Sep 30, 2009 #2


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    Homework Helper

    What is the T?
    It seems to me the only force acting on the top mass is Fn*0.6 so you can solve for the acceleration directly.
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