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Centripetal force ( Swinging a bucket in a vertical motion )

  1. Oct 30, 2010 #1
    First of all,
    the situation is as follows :
    A girl is swinging a bucket filled with water, and she is whirling it around in a vertical motion.
    When the bucket is at any other position on the circumference of the circle, everything seems fine and intuitive.

    But an interesting thing happens when the bucket is at the top of the circle.
    Newton's Third Law implies an equal an opposite reaction due a push or pull.
    At the top, force due to gravity is pointing downwards.
    But what then causes the Reaction force?
    Since nothing is pushing it, where is the cause of it?

    It makes me wonder, if we then increased the angular velocity of the bucket,
    I understand that we need an extra force towards the center of the circle in order to compensate for the extra centripetal force. So the reaction on the water due to the bucket becomes larger.

    What if i tried to explain this using perceived gravity.
    So at the top of the circle, the water perceived gravity as pointing upwards, hence the water will ' BELIEVE ' that the gravity is pointing downwards relative to it and hence will not fall. If that is the case, then it only makes perfect sense to me that the reaction force, now is due to the perceived gravity.
    But is perceived gravity a force, or just what we actually perceive?

    So, to sum everything up, I am confuse about this problem, Can somebody explain what causes the reaction force on the water due to the bucket?
    Many thanks in advance...
  2. jcsd
  3. Oct 30, 2010 #2
    mg = mv^2(at the highest point of the vertical circle)/r
    mg = weight of water in the bucket
    when mg > mv^2/r, then a part of water shall provide the necessary centripetal force; n the rest of the weight of water [mg-mv^2/r] causes some water to accelerate downwards and spill........this is the case when the bucket is whirled slowly
    when the whirling velo is greater then,
    mv^2/r>mg, so all the weight of water prvides centripetal force n the water in the bucket does not spill.

    hope m able to make it clear to u!
  4. Oct 30, 2010 #3
    No object wants to move in a circle. It goes in a straight line or is stationary unless some force is acting on it.
    So in order to move in a circle you must push constantly in direction of the center of the circle to keep it in that motion otherwise the object will continue in a straight line.
    So it the bucket, the water is trying to go in a straight line all the time with the speed you are giving to it, but there is the bottom of the bucket preventing that by applying force to the water.
    So in weightlessness no matter the angular velocity it will stay in the bucket.
    On earth, on the top of the motion, it must create bigger force than the gravity (or equal but then the water will "feel" weightless) or otherwise it will spill.
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