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Styrofoam object- rise or sink?

  1. Dec 11, 2008 #1
    i have a 30 ft pipe - 10" diameter. there is a constant water flow at the top, keeping the water level constant ( the pipe is always full to the top). at the bottom the water just flows out of the basin and away. the pipe is is just 1" underwater in the pool underneath.
    the yellow object is a styrofoam ball - diameter 9" so it will easily fit into the pipe.
    will the ball rise up due to buoyancy and density differencial, or will the pressure from the water column above supress these forces and keep it at the bottom.
    if the ball does rise, will its speed be slowed by the downward flow of water around it?
    what will its estimate spped be, compared to the clasiccla setup where an object is released in a static water column, closed at the bottom .


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  3. Dec 11, 2008 #2
    I could be wrong, but:

    I believe the ball would rise. The water level in the pipe is always the same, so it is as though the ball were placed in a water-filled pipe with a bottom. Therefore there is not a pressure differential because water is added and replaced at the same rate. If the ball were placed at the top of the pipe it would not be sucked downward either.

    As to the velocity of its rise compared to a static column: I believe based on the above that it would be equivalent.

    Great question, BTW.
    Last edited: Dec 11, 2008
  4. Dec 11, 2008 #3
    thanks for the reply.
    anybody else?
  5. Dec 11, 2008 #4


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    Staff: Mentor

    I believe the ball would rise and its speed would be slowed by the water moving past it, but this is a very difficult question to answer mathematically. It isn't an issue of static water pressure, it is an issue of drag.
  6. Dec 12, 2008 #5
    in experimenting with something like this a while back (mine was ping pong balls in tubes)

    the ball will try to rise at some relatively slow speed, based on the buoyancy of the ball and the orifice formed between the ball and the side walls of the tube. the tighter the size of the ball to the tube the slower it will rise.

    now given that rise speed (which will be constant regardless of water column above it) if the water is flowing down the tube faster than that rise speed, the ball will travel down, if the water is flowing slower than that speed, the ball will rise. if the two speeds match the ball will hold its relative position in the tube.

    :) hope this helps
  7. Dec 12, 2008 #6


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    Science Advisor

    I agree with Russ, it's a matter of drag. I imagine that if the drag force was equal to the buoyant force it would not rise. On the other hand if the drag force wasn't large enough it would rise. Dunno for sure...

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