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Calculate the minimum force

  1. Jan 6, 2017 #1
    1. The problem statement, all variables and given/known data
    Diagram.JPG
    ##m=1[kg]##, ##M=5[kg]##, ##u=0.6##
    Calculate the minimum force required for the blocks to remain in the same position thourought the motion. For the part a) there is no friction between block 2 and the floor, for the part b there is and its ##u_f##
    2. Relevant equations
    3. The attempt at a solution

    For the part a) the frictional force between the blocks must be equal to ##mg##. The frictional force is proportional to the force on the block.
    ##mg=F_{fr}##
    ##mg=uN## ##\Rightarrow## ##N=ma## ##\Rightarrow## ##F=(m+M)a##
    ##mg=um\frac{F}{m+M}## ##\Rightarrow## ##F=g\frac{m+M}{u}##
    The book solution is: ##F=mg\frac{m+M}{uM}##
    I was certain i got this right and then i see a different answer in the book. What is wrong?
    For the part b)
    ##mg=F_{fr}##
    ##mg=uN## ##\Rightarrow## ##N=ma## ##\Rightarrow## ##(m+M)a=F-u_pMg## ##\Rightarrow## ##a=\frac{F-u_pMg}{m+M}##
    Is this right so far?
     
  2. jcsd
  3. Jan 6, 2017 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You are assuming that the force between the blocks equals the force applied to the first block. Not so.
     
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