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Homework Help: Friction and blocks

  1. Feb 10, 2008 #1
    [SOLVED] Friction and blocks

    1. The problem statement, all variables and given/known data
    Three blocks are attached by cords that loop over frictionless pulleys. Block B lies on a frictionless table; the masses are mA=6kg, mB=6kg, and mC=18kg. The accleration due to gravity is g.


    2. Relevant equations
    F=ma


    3. The attempt at a solution

    I thought that it would be wise to treat these as two separate problems. I thought I would solve one side by the following. Block B T=ma Block C F(g)-T=ma and thus mg-T=ma
    I thought that to figure out the acceleration of block B (since we know C's is -9.8m/s^2) that i would use the equation a(B)=(m(C)g - (m(C)g))/m(B) But when I do that i would get 0/6. That cant be right because the block C would move because it is heavier than block A
     

    Attached Files:

  2. jcsd
  3. Feb 10, 2008 #2

    mgb_phys

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    A acts in the opposite direction to C so you could think of it as a negative weight on the same side as C. That will simplify it into a single falling block pulling on B.
     
  4. Feb 10, 2008 #3
    Oops, i forgot to mention, i have to find the acceleration of block a. So would i still be on the right track? I feel like my signs are messed up in that equation i had
     
  5. Feb 10, 2008 #4

    mgb_phys

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    Since they are all on the same wire the accelaration of all the blocks is the same.
     
  6. Feb 10, 2008 #5
    so the accleration of block A is +9.8 m/s^2?
     
  7. Feb 10, 2008 #6
    or are you saying that once i figure out the acceleration of one block, then that would be the same for all of them
     
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