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Homework Help: Could someone help me with this AP Physics problem? Thank you!

  1. Jan 4, 2014 #1
    Hi! I really need help answering this question. I am really bad at Physics and have no idea what I'm doing, so if someone could help me out I would really appreciate it! Thank you!

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

    8. A bucket filled with water has a mass of 23 kg and is attached to a massless rope, which in turn, is wound around a 0.050-m radius cylinder that rotates about a frictionless axle at the top of a well. The bucket is raised to the top of the well and then released to fall back into the well. What is the angular speed of the cylinder at the instant the bucket is moving at 8 m/s?

    The answer is 160 rads/s, but how?

    2. Relevant equations

    (I think)

    Torque = Force * radius
    Alpha = [omega (final) - omega (initial)]/time
    Alpha = linear speed/angular speed

    3. The attempt at a solution

    Torque (T) = F (force) * r (radius)
    11.5 Nm = (25 kg)(10 m/s^2) * ( .050 m)

    I have no idea where to go from here or if it's even relevant to the problem :(

    Thanks again in advance.
    Last edited: Jan 4, 2014
  2. jcsd
  3. Jan 4, 2014 #2
    Because the rope is wound on the cylinder, you can assume there is no slipping between the rope and the cylinder. So the speed of the rope is always the same as the tangential speed of the cylinder. Knowing that, and the radius of the cylinder, you should be able to determine its angular speed.
  4. Jan 4, 2014 #3


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    Gold Member

    I think the problem is incomplete. It should also state that there is no slipping between the rope and cylinder. Assuming this to be true, the relative velocity between them is zero. You already know the velocity of rope at that instant. Now think of that equation that relates linear velocity to angular velocity.
  5. Jan 4, 2014 #4


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    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    It's complete.

    Everything is there to answer the question ... plus some extraneous information.
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