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Falling Bucket problem

  1. Mar 17, 2005 #1
    A bucket of water of mass 15.7kg is suspended by a rope wrapped around a windlass, that is a solid cylinder with diameter 0.250m with mass 11.2kg. The cylinder pivots on a frictionless axle through its center. The bucket is released from rest at the top of a well and falls a distance 10.1m to the water. You can ignore the weight of the rope.

    I need to find the Force exerted on the cylinder by the axle.

    I have found so far the tension in the rope while the bucket is falling which is 40.5 N, the speed on impact of the bucket, 12.1m/s, and the time it takes to fall as 1.67s.

    But I have no clue how to proceed to find the force.
  2. jcsd
  3. Mar 17, 2005 #2
    to do this, you m ust have one more piece of information. What I'm seeing here is that there is a constant frictional torque applied by the axle on the cylinder. We could find this out if we had something like the time it took the bucket to fall, or the speed it hit the water with. Then we could see what the difference is between the real situation and the ideal conditions.


  4. Mar 17, 2005 #3
    I dont think I understand what you are telling me to do. And all that information I have found. It is in the original post.
  5. Mar 17, 2005 #4

    Andrew Mason

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    The gravitational force on the dropping bucket provides all the accelerations: the bucket's downward acceleration and the cylinder's rotational accleration (torque = tangential force x radius of cylinder)

    [tex]mg - T= ma = F_{bucket}[/tex]

    [tex]Tr = \tau = I_{cyl}\alpha[/tex]

    [tex]\alpha = a_{bucket}r[/tex]

    With that you should be able to work out the net force on the bucket and T. T is the force on the axle.

    Last edited: Mar 17, 2005
  6. Mar 17, 2005 #5
    ok, I see what you mean. I took the question the wrong way. Do what Andrew Mason said, it is the correct approach. Sorry about the confusion.
  7. Mar 18, 2005 #6
    I took a different approach than what you were telling me to do. Since I had already found the tension on the string, I added that to the Force of gravity exerted on the cylinder and that turned out to be the force that axle was exerting.

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