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Bikini, bucket, and a hose (water displacement)

  1. Jul 21, 2008 #1
    I put on my cutest bikini, grab a bucket that holds one cubic foot of volume, and a 50 pound weight, and jump in the pool. I submerge the bucket, evacuating all of the air, and turn the bucket upside down. I put the weight on top of the bucket. My girlfriend, who looks terrific in her one piece bathing suit, holds a 4 ft length of hose about 2 feet below the bucket and blows into the hose. The air fills the bucket and eventually lifts the weight (one cubic foot of air will lift 62 lb in water)

    Here is our bigger question-

    As we increase the depth, lets say to 100 ft, will air released below our bucket actually deflect from entering the bucket due to greater pressure, or will it displace the water and fill the bucket with the same ease as it does near the surface? There is no need to consider the volume of air needed due to the increased psi at depth. Our concern is strictly regarding the ability of the air to displace the water in the bucket. We wonder if the air will choose the path of least resistance and rise to the bucket, then deflect off to the sides and up to the surface without filling the bucket. Do we have to actually insert the hose up into the bucket now at this depth? If it is true that the greater depth pressure causes a different reaction when the air reaches the base of our bucket, is there a formula to determine this at different depths?


  2. jcsd
  3. Jul 21, 2008 #2


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    At a depth of 100 feet, the pressure will be great enough that you cannot blow hard enough to fill the bucket with air. What will happen is that the hose will fill with water up to the height of the water in the pool. The density of water is 62.4 pounds/cubic foot so the weight of a water extending x feet above one square foot will be 62.4x pounds and the pressure will be (divide by 1 square foot) 62.4x pounds per square foot. (Technically, you should add atmospheric pressure to that but since there is also atmospheric pressure on your lungs they will cancel.)
    According to this
    http://web.media.mit.edu/~testarne/TR328/node3.html [Broken]
    we can exert only about 2 percent more than atmosphic pressure with our breath; that is, about 3 pounds per square inch. You don't have to go down very far before the water pressure is much more than that. That's why you can't go down much more that 5 to 10 feet before it becomes impossible to breathe though a tube to the air.
    Last edited by a moderator: May 3, 2017
  4. Jul 21, 2008 #3
    In this instance a compressor would be used. Getting the air to the depth is not the issue here. My question is more focused on the reaction of the air once it is rising at that depth, but thank you for the reply. As stated "Our concern is strictly regarding the ability of the air to displace the water in the bucket."

  5. Jul 21, 2008 #4
    It would just take more air to fill the bucket, right? since the pressure at 100 ft is about three times atmos, it would take three times as much air before the bucket was full (or, empty, depending on how you look at it). But I think the bubbles would rise from the hose into the bucket just like you see closer to the surface. Then once the bucket is full of air, the bubbles would start to spill out around the edges. It you removed the weight, the bucket would rise, spewing bubbles as the air expanded.
  6. Jul 21, 2008 #5
    That is what I hope, but in a breakfast discussion on Saturday, someone that has a greater intellect than I (physics dept of U of Utah) told me that the air at that depth would not enter the bucket (we were specifically discussing a 5ft diameter sphere cut to resemble a deep bowl), but would move to the side and rise.

  7. Jul 21, 2008 #6


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    Not enough information. Please elaborate on the parts concerning the bikinis. This is important. Photos are best.

    See attached diagram. Have I misunderstood either of the two viewpoints?

    The air most certainly would rise into that container. You are correct, he is dead wrong.

    Hm. He can't be that dumb can he? Is there any possibility you guys are seeing the setup differently?

    [Also, we will need to test HoI's claim that your gf cannot blow hard enough.]

    Attached Files:

    Last edited: Jul 21, 2008
  8. Jul 21, 2008 #7
    Thx for the neat gif. Yes, that is what he was explaining and yes I disagree. in a larger explaination of the discussion, we were talking about a roller chain system (think bike chain but vertical not horizontal) where you have an upper shaft and lower shaft with the chain attached. Say that the torque required to rotate both shafts was 22,000 ft lbs of torque and you attached these "air lift bowls" along the chain at certain intervals. If you introduce the air into the bottom of the tank of water, factoring the pressure at depth and introducing a given amount of air (fill container slightly, maybe 1/5) you could get enough lift from the air to rotate the system. If you took a snap shot of the system and calculated the given lift for each container depending on its depth and expanding air volume, you could get a desired amout of constant lift (say 30,000 ft lbs). No great plan for such a system, just a general discussion, but when he tossed out the opinion that the air would not enter the container, I disagreed. After thinking about it though, I countered with an idea of adding an "air box" where the air was introduced and using the growing pressure create an intermitent "air burp", then a mechanism that would allow the bottom of the air lift containers to be opened then closed just as the air entered the container. A bit more complex but able to solve his problem I thought.

    Summer pic included, funfun !!!

    Last edited by a moderator: Jul 21, 2008
  9. Jul 21, 2008 #8


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    I'm not sure: is your modification supposed to fix the problem he mentions? cuz the problem he mentions is bogus. There's no need to fix.

    If your mod has other uses you haven't mentioned then OK...
  10. Jul 21, 2008 #9


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    Of course you can fill a returned bucket with air underwater. It is a trick divers use to transport heavy stuff to the surface, and they call it a "parachute". It's simply a strong plastic bag with ropes on it, which you attach to the thing you want to lift (say, an anchor). Then, with the bag opening downward, you let air (from your breathing apparatus) rise in the bag. It gets filled, and it gives you an upward force which equals the weight of the removed water from the bag: if you have a bag of 50 liters, then you can rise 50 kg with it. It is amazing: a small "pssst", and you can lift things which you cannot even take up with your hands.
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