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Blocks and pulleys

  1. Mar 28, 2004 #1
    Can any of you help me in how to treat the cables in the arrangement pulleys and blocks attached ?.

    thanks, Leonel

    Attached Files:

  2. jcsd
  3. Mar 29, 2004 #2
    Is cable E fixed to the ground on one end, by that little parallelogram block?
  4. Mar 29, 2004 #3


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    Assuming that the cable is of fixed length and fastened at its end, then the length of the cable, E, is the distance from that fixed point to block B and so its length is increasing at 18 in/sec just like the blocks motion. Also, it is clear that, since the two pulleys can be taken to be right up against blocks A and B, C and D are of equal length at all times and so have the same rate of change.
    Specifically, C+ D+ E= fixed length so dC/dt+ dD/dt+ dE/dt= 0. dC/dt= dD/dt and
    dE/dt= 18: 2dC/dt= -18 or dC/dt= dD/dt= -9 in/sec.

    Since the distance between blocks A and B are decreasing in length by 9 in/sec, A's speed relative to B is 9 in/ sec: yes, A's speed relative to the stationary base is 27 in/second.
  5. Mar 31, 2004 #4
    That's correct, Chen !
  6. Mar 31, 2004 #5
    HalsofIvy, thanks a lot for your response ! My thoughts have been the same of yours..

    But, the problem is that the book responses are: cable D Velocity is assumed to be + 36 in/s while the Vcd is - 18 in/s. therefore, that makes Vc = + 18 in/s. What results in a complete difference...Maybe some typing problems (2 mistakes ??!!) in the book "Vector Mechanics for engineers" Beer and Johnston" fifth ed. Ex.11.42.
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