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Density of water

  1. Feb 9, 2010 #1
    Today on my way to a swimming lesson, i wondered why swimmingpools with other kinds of liquid mass in them havn't been invented? I was wondering if water with a different density and lighter mass wouldn't equal that a human beeing would more easily go through the water and not hit himself as bad if he trains jump-ins and such. Why havn't a lighter mass watertype been invented? Wouldn't it be perfect for huge jump training so you wont hit yourself as bad. A downside would ofcourse be (I believe its Newtons law) that you wont swim as fast because of friction (action/reaction) would be less.

    Any way, just a thought :) New to the forum. Hello all.

    Ps: Please bare with me. Im not a physics major or anything classy, just interested.
  2. jcsd
  3. Feb 9, 2010 #2
    One immediate problem would be sinking. If the liquid density were half of water (half a human), then the swimmer would have to produce an upward thrust of ~ 50 Kg just to stay on top.

    Bob S
  4. Feb 9, 2010 #3
    so thats about 25 kg in normal water?
  5. Feb 9, 2010 #4
    According to Archimedes, if your normal weight (mass) is say 80 Kg, your weight in regular water is ~0 Kg, and ~40 Kg in half density water. So you will sink.

    Bob S
  6. Feb 9, 2010 #5
    Right. What if your weight is 40 kg then? 0 and 20 in the other water?
  7. Feb 9, 2010 #6
    40 Kg in regular water and 20 Kg in lite-water. The buoyant upward force is equal to the weight of the water your body displaces.

    Bob S
  8. Feb 9, 2010 #7
    but that makes no sense? You are way easier to lift in water? So you have to at least feel lighter? If you weighed 100 kg it feels like 50 in water if a persons lifts you etc?
  9. Feb 9, 2010 #8


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    Right. In your substance X, rather than weighting 80kg, you might weight 40. You would have to geenrate 40kg of lift while swimming to stay afloat.

    Think of your substance as somewhere between air and water.

    You have a tiny amount of bouyancy in air. It will take 80kg to lift you.
    You have a lot of bouyancy in water. It will take 0 kg to lift you.
    You have a small amount of bouyancy in substance X. It will take 40kg to lift you.

    BTW, one of the ways of making such a substance is to inject it with air bubbles. People fallen into vats of foam (such as processing beer). They sink to the bottom and drown because the substance is not dense enough to swim in.
  10. Feb 9, 2010 #9
    That's a great post. I understand you now. Ill just look up bouyancy to grasp the term ;) But in generel I get your point.

    Back to topic though. Why don't you make water that requires 20% of your bodyweight to lift you. It would be able to swim in, and not hurt when you land in it?
  11. Feb 9, 2010 #10


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    Staff: Mentor

    By the way, this:
    ...is a self-contradiction. A swimming pool with a fixed volume of water has one mass and an associated density. If you increase the mass, you increase the density and vice versa. You can't increase the density while decreasing the mass unless you make the pool smaller and the making the pool smaller bit doesn't have any effect on buoyancy or drag.
  12. Feb 9, 2010 #11


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    Because you'd sink. And then drown.

    Try this: tie 35 pounds of rocks around your waist, jump in and try to swim.

    Hint: Don't.
  13. Feb 10, 2010 #12
    You're right. Probably would be too hard to stay afloat.
  14. Feb 10, 2010 #13


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    There are no "waters of different density". Density is a property of the substance, so water has always the same density. You can slightly change the density of water dissolving something - for example ammonia solutions have lower density, while kitchen salt solutions have greater density (which is why it is easier to float in the sea than in fresh water). Trick is, you will not want to swim in most of these solutions :yuck: One can think about using different liquids, but in most cases they they will be either expensive or poisonous (or both).
  15. Feb 10, 2010 #14
    Right - you could fill a pool with mercury, and barely get your knees 'wet'...
  16. Feb 10, 2010 #15


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    Actually, when I first saw the title of this thread, I thought you were talking about using liquids denser than water so it would be easier to swim in them! Of course, a major problem with all of this is that all water has the same density so you would have to use other kinds of liquid- and they are almost uniformly poisonous (many if absorbed through the skin).
  17. Feb 10, 2010 #16


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    There are of course already "different types" of water; e.g. very salt water like in the Dead Sea (density 1.2x that of water) or water with continuously "injected" air (lower density).

    It is also possible to make water more "slippery" by mixing in a small amount of certain polymers, although I've never heard of anyone actually filling a whole pool with that stuff.
  18. Feb 11, 2010 #17
  19. Feb 11, 2010 #18
    The salty dead sea water was a good example :)
  20. Feb 12, 2010 #19
    Last edited by a moderator: Sep 25, 2014
  21. Feb 12, 2010 #20


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    Seems to me like they have mostly changed the viscosity, not the density.
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