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Circular motion of bucket of water

  1. Sep 2, 2003 #1
    when a bucket is filled with some water, you take it up and swing it around, if you swing it hard enough, the water in it would not spill out, why is this so?is it because the reaction force from the centre of rotation is acting on the water?
  2. jcsd
  3. Sep 2, 2003 #2


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    In order to move the bucket out of a straight line, you have to apply force to it (this is the "centripetal" force). If you swing it fast enough, the force you are applying to the bucket (and so the 'reaction force' as you put) will be greater than the force of gravity.
  4. Sep 2, 2003 #3
    To calculate this force you might need this:

    F = m*v^2/r

    F = the centripetal force (i don't know if this is a correct word )

    m = mass

    v = velocity

    r = distance
  5. Sep 2, 2003 #4
    If that didn't quite clarify it enough:

    When you swing the bucket in a circle, there is a force pulling toward the center of rotation, which would probably be at your shoulder. When the bucket is above your head, that force is pulling downward. If you are swinging fastly enough, the force generates acceleration that is greater thant the acceleration due to gravity. So the water cannot escape from the bucket due to gravity, because the bucket is accelerating downward faster than gravity is causing the water to. It's like trying to run away from an olypmic runner. Basically, as long as the water and bucket are putting a substantial amount of pressure on each other, the water won't fall out of the bucket.
  6. Sep 3, 2003 #5


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    Another way to explain it: When the bucket is at the top of the rotation facing downward, the water is accelerating down out of the bucket. But your force exherted on the string is accelerating the bucket down FASTER than the water can accelerate out of the bucket. So the water stays in the bucket.
  7. Nov 24, 2003 #6
    I just wonder...

    In the bucket you are swinging around you put a piece of wood,
    if the bucket is above your head, will the wood be at the same level
    in the water as it was in rest ?

    this question was asked during a science quiz here in Holland.
    Really nobody knew the answer, nor had any idea, maybe one of you?
  8. Dec 17, 2003 #7
    I would think the block would stay at the same level. The change in acceleration would be analogous to changing the acceleration due to gravity.
  9. Sep 22, 2004 #8
    The reason why the water does not leave the bucket is because the water is wanting to travel in a straight line but is being forced to turn with the bucket because it has no where else to go. Just like when you are on a rollercoaster and doing a loop. Say you are traveling, with a ball in your hand, in a straight line going pretty fast and were about to do a loop and just before you went up to start the loop you let go of the ball. The ball would keep going in a straight line as you were turning up and it would seem as if the ball fell to your feet. This is why...
  10. Sep 23, 2004 #9
    The water doesn't fall because of the same reason that the moon doesn't fall to the earth. Even though it is being pulled by it.

    Due to the velocity that it has, at every point in its motion, it deviates from the path of falling, and due to the force of gravity, from following a tangential path.

    And what keeps the bucket from flying off is your hold on it.

  11. Sep 25, 2004 #10
    The definitive accommodation of your question can be answered in a recording by Dr. F. Zappa where he not only describes centripetal motion and it's local effects , but also is the first noted mention of string theory, The essay is entitled ," Moving to Montana soon".

    The man died rather young after contracting probate cancer by exposure to Tipper Gore,
  12. Oct 8, 2004 #11
    Hi..my question is on finding the period for the moon's motion around the earth. I need to express my answer in days and compare it to the length of a month. The moon orbits the earth at a distance of 3.85 X 10^8m. The mass of the earth is
    5.98 X 10^24 kg. I know that I am supposed to use the formula T=2 Pi (r^3/2) / square root of GMe, and then when I get my answer for that I divide by 86400 to express my answer in days. I think my answer should be somewhere around 27.153 days, but I am not getting anything close to that. I have worked and worked the problem out and I just cannot solve it. I think I may be using my calculator incorrectly. I am coming up with an answer of 2.68^11 and then when I divide that by 86400 I get 3101851.852, which is very wrong. Please help!
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