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Homework Help: Stacked blocks with friction

  1. May 3, 2012 #1
    1. The problem statement, all variables and given/known data
    A block of mass 3kg sits on top of a block of mass 2kg. The static coefficient of friction between the blocks is μs=0.3, and this friction keeps the top block from sliding on the bottom block. There is no friction between the bottom block and the floor. A horizontal external force Fext is applied to the bottom block, causing the blocks to accelerate to the right.

    How does the magnitude of the total force acting on the bottom block, Fb, compare to the magnitude of the total foce acting on the top block, Ft?

    a. Ft>Fb

    2. Relevant equations

    None given but μs*N=Ffric and F=ma.

    3. The attempt at a solution
    I know that all of the forces in the y direction cancel but im not sure how to tell which force is greater? I know that the answer is A but im having trouble explaining it or showing work.
  2. jcsd
  3. May 3, 2012 #2


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    In the y direction there is no acceleration of either block, thus, there is no net total force on either block in that direction. You should focus instead on the horizontal forces acting on each block. Since they both accelerate together at the same rate, which block must have the greater net total force acting on it in the horizontal direction, and why?
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