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Homework Help: Pulley w/ attached weights/Linear+Angular acceleration.

  1. Nov 25, 2006 #1
    Determine the linear and angular acceleration of the disks.

    A diagram is illustrated as having a small disk (radius 1m) with a rope hanging on its edge with a mass of 12 kg attached to it. Another pulley disk (radius 2m) surrounds the smaller and first one; it has a rope attached to a 4 kg mass.

    This is what I have so far:

    T1 - m1g = m1a1
    = t1-117.6 = 12a1

    m2g - T2 = m2a2
    = 2 [39.2 - T2 = 4a2]
    = 78.4 - 2T2=8a2

    In order to find the accel., I calculated the velocity from the radii and distance using arc length.

    d2= 2(theta)
    d1= 1(theta)

    d2/d1= 2(theta)

    a(tangential)= (alpha)r (alpha= angular accel.)
    (alpha) = a1

    T1 - 177.6 = 12a1
    78.4 - 2T2 = 8a2=8*2(alpha)=16(alpha)
    + 2T2 - T1 = 4a1

    -39.2 = 32a1
    a1= -1.225

    a2= 2(-1.225)

    Having done that....are the objects decelerating not accelerating? And have I completed all the steps correctly?
  2. jcsd
  3. Nov 26, 2006 #2
    can you show the information more clearly? i can't think of the picture
  4. Nov 26, 2006 #3
    I hope this helps!

    Attached Files:

  5. Nov 26, 2006 #4


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    You may have gotten the right answer, but you certainly did not do things "correctly". When you do not use units it makes it very difficult to follow your work.

    If the system starts from rest it is speeding up. Whether the accelerations are positive or negative depends on the directions you chose to be positive when you set up your equations.
  6. Nov 27, 2006 #5
    Assuming that up and right (y) are postive and down and left are negative (x), so the answer would still remain negative. Would that indicate that the object on the left (12 kg) is moving upwards while the object on the right (4 kg) is moving downwards? If the acceleration of both are negative wouldn't that indicated that both are moving in the same direction?
  7. Nov 27, 2006 #6


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    In your original equations

    T1 - m1g = m1a1
    = t1-117.6 = 12a1

    m2g - T2 = m2a2

    you identified the 12kg mass as m1 and the 4kg mass as m2. Your first equation is written as positve upward. Your second equation is written as positive downward. This is a reasonable choice, since one mass will move upward and the other will move downward. Your negative answers for both accelerations just means that in fact the 12kg mass (m1) will accelerate downward while the 4kg mass (m2) will accelerate upward. The magnitude of the static (i.e., pulleys held in place) CCW torque, 12kg*1m*g, is greater than the magnitude of the static CW torque, 4kg*1m*g, so these directions are to be expected.

    I don't see how you got your answer. Do the disks have mass? Is there a moment of inertia given?
  8. Nov 27, 2006 #7
    I apologise, I left out the fact that I = 4 kg m^2 .
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