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Submerged in mercury

  1. Mar 14, 2013 #1

    I’m a non-scientist looking for help with a physics (I think) question regarding a mystery I’m writing.

    A man is submerged in a very small pool of elemental mercury. He is strapped to bolts at the concrete bottom, so that he will not float. The liquid just covers his body.

    My question: given the weight of mercury, will this harm/crush him?

  2. jcsd
  3. Mar 14, 2013 #2
    The density of mercury is 13 times that of water, so the pressure is as large as if the person is submerged 13 times as deep into water. So for the persons feet the pressure is as high as 20m deep in water. At the breast where the pressure is on the lungs the equivalent water depth is maybe 0.4 m*13=5.2 m

    The person will be in a lot of pain though because the buoyancy has a force of 6kN that's more then half a ton of weight pulling on the feet. (Weight of the submerged part of the body*12)
  4. Mar 14, 2013 #3
    I rather think the man would die or atleast have some severe symptoms from mercury poisoning than some physical crushing in that depth.
    @BEEF yup you were faster.While I search for links someone usually takes the lead. :D
    Mercury is a kinda dangerous element , it's toxic to humans and as much as I know most other animals too.
    Although mercury vapor is more dangerous than the liquid.Anyways it does alot of bad things to the CNS system and other organs.

  5. Mar 14, 2013 #4


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    Because he's so buoyant? That's actually kind of bizarre.
  6. Mar 14, 2013 #5
    @ crazymechanic That is different. The pressure doesn't hurt the person, I thought this became clear from the examples. It will probably not even cause a lot of breathing problems, maybe some trouble with the blood circulation in the legs, because of the pressure differential. The buoyancy force is the weight of the displaced fluid on the other hand. It is half a ton for the mercury. In water the weight of the diver, and the buoyancy force cancel almost completely, this is why divers experience a kind of weightlessness, but the mercury guy has half a car hanging on his legs.

    If you guys have a different result let me know!
  7. Mar 14, 2013 #6
    Thanks for the replies. I really appreciate the help.

    Elemental mercury is poorly absorbed through the skin, and the Hg would only very slowly release vapor, so poisoning isn’t the immediate worry.

    So if I understand the info so far correctly:
    The pressure on the lungs is not going to be a big problem for the guy.
    However, the bouyancy will be a problem. (not sure I fully understand this bit, sorry).

    What if the guy’s torso is strapped down but his arms and legs are not, and so float freely?
  8. Mar 14, 2013 #7


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    I think this man's biggest problem is that there's someone out there willing to submerge him in mercury. So if the mercury doesn't get him, something else will.
  9. Mar 14, 2013 #8
    @cayucos It doesn't matter. The pull on the person would still be half a ton as long as most of the body is covered, unless the barrel or whatever container is so small that the level changes a lot when the person gets submerged so less liquid needs to be displaced.
  10. Mar 14, 2013 #9


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    @0xDEADBEEF: If only his torso were tied down, then maybe he'd flip over with his legs out of the mercury, and there would be a significantly less buoyancy force on him than in the "upright" scenario. And then we're back to the "can't breathe" problem. :tongue2:
    Last edited: Mar 14, 2013
  11. Mar 14, 2013 #10
    Ah, gotcha. If you’ll indulge me a bit further:

    I can make the container small enough that it just accommodates the man’s body (fiction is useful, that way ;). So, about six feet long and a couple of feet wide. Would that do the trick?

    He can be in some pain but not so much he cannot talk, or respond to questions. I’d prefer as little pain as possible, for purposes of the plot.

    Yup, the person who put him in this position in the first place is not a nice person.
    And, no, he can’t flip over, because I will not allow it.
  12. Mar 14, 2013 #11


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    @cayucos: So the problem is that however he's being held down, it will have to hold him with enough force to overcome buoyancy. If he's lying on his back, held down by a web of straps in his "coffin" of mercury, then the straps holding down his torso are going to have to be able to overcome the buoyancy of his torso, the straps holding his legs will have to deal with his legs, and so on. Maybe his torso is only 1/4 of his total buoyancy, but it still seems like it would be hard for him to talk with the equivalent of a 250 lb weight on his chest.
  13. Mar 14, 2013 #12

    Arggh. Doesn’t leave me much wiggle room. Okay, back to the drawing board:

    Perhaps I can tweak this scene so that he is in the container, partly submerged. And the THREAT is to add more mercury to the container, covering him, causing all manner of discomfort.

    So let’s say he’s lying in a XX-inch depth of Hg; strapped down. Meaning the top half of his body is not submerged. Am I in business with this scenario?
  14. Mar 14, 2013 #13
    maybe just electrocute the guy with high voltage low amp DC ? :D:D
    Speaking of causing pain....
  15. Mar 14, 2013 #14

    Actually, I do have a lightning storm in the wings, and the elemental mercury is metallic...

    Sigh. Really do want to keep my guy in that mercury bath. And have him capable of speech....
  16. Mar 14, 2013 #15
    Why don't you put the person below a grid. He will float in mercury and you can press him against the grid with more and more force by adding more and more mercury.
  17. Mar 14, 2013 #16
    Now that's clever. So, if the mercury level rises enough he'll be pressed against the grid, with the threat of, well, drowning in the stuff. I assume drowning would come before the issue of pressure arises.
  18. Mar 14, 2013 #17
    In fact, just went to sketch out the scene with the grid idea and I like it. It works. Thank you!

    In fact, thanks to all here who've given great ideas and some laughs. You physics geeks rock. (using 'geek' admiringly)
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