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Homework Help: Rotation about a fixed axis problem

  1. Jan 9, 2012 #1
    Hi guys I was trying not to post this as it is difficult to understand without the diagram, but I will try and give you a visual. Basically there are 2 pulleys attached together on the one pivot, the larger one is driven by a belt that is driven by a motor. The inner pulley is used to hoist the weight.

    1. The problem statement, all variables and given/known data
    Figure 3 shows a block A attached to a hoisting mechanism which consists of two
    drums of radii r1 = 0:4 m, r2 = 0:8 m, ¯xed at O and which have a combined mass
    of m1 = 100 kg and a radius of gyration about O of kO = 0:5 m. If a constant force
    P = 1:5 kN is applied by the power unit, which consists of a pulley of radius r3
    attached to a motor, determine the vertical acceleration of block A. The cord from
    the motor to the drum is at an angle 40± to the horizontal. Block A has a mass
    m2 = 300kg.

    2. Relevant equations
    I = mk2

    +CCW [itex]\Sigma[/itex]Mo = Io[itex]\alpha[/itex]

    +[itex]\uparrow[/itex][itex]\Sigma[/itex]Fy = m(ag)x

    +CCW a = [itex]\alpha[/itex]r

    a = [itex]\alpha[/itex]r

    3. The attempt at a solution
    I = 100kg * (0.5m)[itex]^{2}[/itex] = 25kg.m2

    T(0.4m) = (25 kg.m2[itex]\alpha[/itex]

    +[itex]\uparrow[/itex][itex]\Sigma[/itex]Fy = m(ag)x
    -300(9.81)N + 1500sin40N + T = - 300a

    a = [itex]\alpha[/itex]0.4

    I worked out [itex]\alpha[/itex] to be 5.56, or 24. something. Basically after 20 odd hours, I have given up, there is something clearly very wrong and missing, and between me and 2 other people we don't know where to go from here.

    The answers are as follows
    [[itex]\alpha[/itex] = 0:312 rads/s2CW, T = 2980:5 N, a = 0:125 m/s2.]

    Please help.

    Kind Regards

    Rob K
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jan 10, 2012 #2
    Can anyone help with this?

    Do I need to provide some more information?
  4. Jan 12, 2012 #3


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    Homework Helper

    Try to attach a picture. And explain your notations. What do you mean by ":"? Is it for decimal point?

    Last edited: Jan 12, 2012
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