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Homework Help: ME Dynamics - Multiple ideal pulleys + an inclined plane

  1. Feb 14, 2015 #1
    1. The problem statement, all variables and given/known data
    Find the acceleration of both blocks
    2. Relevant equations

    3. The attempt at a solution
    Everything in black is given in the problem. The red/blue is my work.

    First I want to relate the acceleration of block A to block B by finding the constraint equations:

    Lrope = 2L1 + L2 + 2L3 + constants(lengths around pulleys)

    taking the derivative twice to find the acceleration:

    0 = 2L1 + L2 + 2L3

    I feel like I have too many variable here. I should be able to knock it down to two so that I can relate the acceleration of A to B, correct? How would I do that?

    The forces on B, all in Y using the normal xy-coordinate system.

    MbAb = 3T - Mb(g)

    The forces on A, using a coordinate system along the incline where n1 is "up" and n2 is positive along the slope DOWN:

    Sum of the forces in n1 = ma(n1) = Na - mgCos30
    Sum of the foces in n2 = ma(n2) = -2T - Fa + mgSin30

    Relating the two coordinate systems:

    n1 = Cos 30 j - Sin30 i
    n2 = -sin30 j - cos30 i

    Have I done everything correctly so far?
  2. jcsd
  3. Feb 14, 2015 #2
    There's a small error in the drawing. The small pulley right above L3 is fixed to the large pulley above it.
  4. Feb 14, 2015 #3


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    Staff: Mentor

    To investigate your constraint equation, consider the situation if block A moved upslope by some distance x. Two strings are "giving up" a length x, so that makes a length of 2x available to pass over the top pulley. That "new" length has to be distributed over how many lengths as the bottom pulley moves down? By how much must each extend in order to accommodate the "new" 2x?
  5. Feb 15, 2015 #4
    They must extend by 2x/3 ?
  6. Feb 15, 2015 #5
    They must extend by 2x/3 ?
  7. Feb 15, 2015 #6


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    Staff: Mentor

    Looks good.
  8. Feb 15, 2015 #7
    So then

    a1 = 2/3 a2?
  9. Feb 15, 2015 #8


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    Staff: Mentor

    Well, depending upon which block is a1 and which is a2, that seems to be a valid conclusion.
  10. Feb 16, 2015 #9
    Ops :). a1 would be the acceleration of block A and a2 wold be the acceleration of block B.
    Last edited: Feb 16, 2015
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