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Homework Help: Circular Motion (twirling of a ball on a string)

  1. Jun 26, 2009 #1
    1. The problem statement, all variables and given/known data

    "A brick is in a bucket. The professor twirlings the bucket around his head in a circular motion. The moment that bucket is directly over his head. The tension is zero. What is the acceleration of the brick at this given moment?

    I narrowed it down to two answers.
    a) The tension cannot be zero
    b) the acceleration is 9.8 m/s^2 DOWN

    3. The attempt at a solution

    I believe the tension cannot be zero because if there was no tension in the string, it would no longer continue on its circular motion.
    The only thing that bothers me is the acceleration. I understand that the acceleration is proportional to the force, which would be the centrifugal force (centre seeking), so it would of course be down.

    I asked the professor during the exam, "Don't both answers go hand in hand? are there not two answers for this question?". He said the question still stands. SO URRRRRRRRRRRG

    Any help would be appreciated!
  2. jcsd
  3. Jun 26, 2009 #2


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    Gold Member

    Assuming the bucket is undergoing uniform circular motion, the tension in the string can be zero the moment that the bucket is above the professor's head. Think of it this way: All of the bucket's velocity is directed in the horizontal direction at this point, so when it begins to fall vertically from this height, it then encounters resistance from the tension in the string; consequently, we can let the tension in the string be zero at this point only.

    Since the tension is zero (only gravity is acting) and the bucket is moving in a circular orbit, we know that the bucket's acceleration is directed towards the center of the circle. Since only gravity acts, what does this suggest about the bucket's acceleration?

    Sorry if this is incomprehensible.
  4. Jun 26, 2009 #3

    Doc Al

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    Staff: Mentor

    What's required for circular motion is a centripetal force, not necessarily from string tension. What other force is available to provide a downward force?
    That's centripetal, not centrifugal. (Centrifugal means "away from center".) What provides that force?
    You are told that the tension is zero. Use that information, but don't contradict it.
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