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Homework Help: Designing/Finding Something that Sinks Slowly

  1. Mar 31, 2013 #1
    I need to create an object that will sink 100 cm in 100 seconds. The closer, the better. This means that it needs to be sinking at the rate of 1cm/second, if it is constant (although it is not possible), and it needs to be just slightly more dense than water.

    I've tried a water bottle with rocks and water in it, but it increases in momentum too fast and too much (not sure if that was the correct term). It begins at the rate of .5cm per second, but then after about 10cm, it goes at the rate of 3cm/second. If I remove the slightest of water (just a drop), it floats.

    I also tried a glass container, and kept adding small rocks (very small). However, the same thing happened. The glass container started off sinking very slowly, but then picked up speed very fast. I know that is how gravity works, but how else am I supposed to accomplish this? Perhaps the shape?

    Carrying on, I attached a CD on the lid of the glass container, with the water and the rocks. I was able to get to about 60cm in 45 seconds, but when I tried again, it sank in about 20cm...

    My next idea is to use a balloon, and filling it up with water, and adding/subtracting air.

    Any ideas? This project is simply a HIGH SCHOOL experiment challenge, so it may not be very difficult.

    Thank you.
    PS, I am not sure if this is in the correct section (perhaps the Physics?). Would be appreciated if someone would come to aid.
  2. jcsd
  3. Apr 1, 2013 #2
    You have several problems with trying to balance air and weight.
    The air compresses and get less buoyant.
    Think about something like a polypropylene with a SG of .94.
    It floats low in the water, because of it's SG, add a little weight, to offset.
    Read up on slow sinking fishing lures.
    I think the weebles toy is made out of PP.
  4. Apr 1, 2013 #3
    Try a soda bottle sand put four holes around the circumference ant up the side at 2cm intervals it should sink slow enough also put play dough (us equilevant) at the bottom for constant weight look up ballest in a submarine for more information
    Last edited: Apr 1, 2013
  5. Apr 1, 2013 #4


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    Staff: Mentor

    Welcome to the PF.

    Could you post the exact problem statement from the project assignment? What constraints are put on the device? Can it be intelligent and powered?
  6. Apr 2, 2013 #5
    Some dimensions of your device would be helpful for the following conjecture. I suspect a problem might be the fact that your hydro static pressure is increasing with increasing depth thereby increasing the acceleration as you descend below the water. This would only be true for tall, slender containers though. I am not sure if my reasoning is correct on this one.

    Another problem may be that as you decent closer to the surface the momentum of the water in front of the body will tend to move around it. more simply the vector field direction will be less normal to the surface of the object. As you get close the floor you get a pseudo "casimir effect" which will "suck" the object quickly to the surface. This is caused by a pressure differential between the top and bottom of the body. Look up some basic fluid mechanics. That may help you get started.

    I suspect a small part of your problem comes from the shape of your design. The highest drag shape would ideally be disc-like (large surface area. small width), with a low aspect ratio (low ratio of length to height) with sharp corners.
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