Why Do People Float On Water When Dead and Sink When Alive?

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  • 1 decade ago
    Favorite Answer

    as bacteria start to work and decompose the bodies internal tissue - gas builds up which reduces overall denisty of the body - so the body floats.

    The body actually should float - if you got proper swimming lessons - the key is archimedes principle - ie the more water you displace creates upthrust to the mass displacing the water - so it floats -this explains why iron ships float. However, if you lose of centre of gravity by moving about (agitated) you are liable to sink - so try to relax when you try to float.

    BTW - on the dead sea - everyone floats - why the highly concentrated salt water is more dense than the body...

    Thought you might like to know that one

    Source(s): learnt this in school long long long time ago
  • Kes
    Lv 7
    1 decade ago

    Most live people displace about as much water weight as they weigh and they just barely sink or swim. Less people would drown due to panic if they realized that they could be supported with their nose above water by a simple piece of yarn and can therefore use their hands easily to tread water or to dog paddle to shore. The secret is to not try to keep the entire head out of water and to breathe between waves. Women have more natural body fat then men and can usually float motionless. Men can often float motionless on their backs by arching their back and holding a large breath of air. They may slip a couple inches under water after gasping in a new breath but will resurface.

    That said, it takes very little buoyancy to cause a dead body to float even if shoes and clothing drags it down. A small buildup of gas from bacterial action in the gut or from decomposition is often enough. A well bloated body can even bring up a weighted body, hopfully leading to an arrest.

  • 1 decade ago

    The exact floating cycle for someone who dies is: first they sink, then they float as they start to decay - this is not very long after they first sink. Eventually though, they'll sink again.

    Found this site quote to support that:

    "Yes and no. Or rather no and yes. Or rather, first no, then yes, then no. A newly dead human being, or any other animal, will sink when placed in water. After the gases of decomposition build up in the chest and abdomen, however, the body will inflate, rise like a hydrogen-filled balloon, and pop to the surface, sometimes dragging with it surprisingly heavy weights that a murderer might have thought sufficient to keep it down. With the passage of time and further decomposition, however, the body cavities eventually rupture, the gas escapes, and the corpse goes down again, this time for good.

    There is one notable exception: sometimes, due to air caught in the clothing, a body will stay on the surface for several hours before sinking."

    Of course this is the simplistic answer. Some bodies found in water do not go through that first sinking. Here's some factors found on the straightdope website:

    " * Lungs. Lungs are like a sponge. When someone drowns, the air sacs in the lungs fill with water. Since a body without air in the lungs is denser than water, it sinks. A person who is killed on the surface and then put in the water tends to float, since the lungs are still full of air (although see below). That's how pathologists can tell whether a person was drowned or was dead before hitting the water.

    * Body position. A person who was dead before entering the water can still sink depending on the position of the body. If the body is upright when dumped into the water, water can enter the lungs while air escapes. Hence, the body sinks after a short time. If the body is prone (face down), the air in the lungs can't escape, so the body floats.

    * Body fat. Body fat is less dense than water. The fatter a person is, the more buoyant the body. Muscle on the other hand is denser than water, so people with a lot of muscle--or people who are just plain lean--tend to sink.

    * Clothing. Some fabrics trap air well; others don't. Natural fibers, like cotton and wool, absorb water and so tend to sink. An exception is silk, whose fine fibers can be woven tightly enough to trap air. Some synthetic fibers, like nylon and polyester, usually don't absorb water, so they can trap air. Other synthetic fibers, such as orlon and other fibers used in cold-weather clothing and sleeping bags, are manufactured like some pastas, with one or more tunnels in them. These tunnels are designed to trap air and insulate the wearer. These fibers float until they fill with water, at which point they sink like a stone.

    * Putrefaction. A body that sinks doesn't necessarily stay sunk. As the corpse decays, it generates gases that collect in various body cavities. That's why corpses become bloated, whether in the water or not, giving us the charming term "floater."

    "

  • Anonymous
    1 decade ago

    You know, I answered excatly the same question just a few hours ago. Do I smell a homework question.

    First off most people don’t sink when they are alive. Only very thin people or very muscular people will sink when they are alive, and even then they need to breath out to do it. Most people have more than enough fat on their bodies to keep them lighter than water, and they float.

    Some people sink when they drown for the same reason that thin or muscular people sink when they breathe out. The air in the lungs acts like an inflatable swim toy and bouys people up. When people drown they breathe water into their lungs. With a lung full of water the body becomes more dense and in many cases dense enough to sink. Note however that even moderately fat people won’t sink even when they drown. Fat and oil are much less dense than water and keep them bobbing to the surface.

    Some bodies will initially sink when they die and will then rise to the surface again. That’s because the body starts to rot. When it rots the bacteria produce gases like methane and carbon dioxide. These fill up the skin like a balloon. Since the body is now full of gas it bobs to the surface like beach ball.

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  • Debra
    Lv 4
    4 years ago

    The only sensible answer here is from Kunwarrarra. All the other answers are wrong. However, if you do have the urge to jump off a boat you might first consider the weight of the clothing and other attached bits & pieces. As for normal swimming, you will float well enough and, depending upon how fat you are, you could happily relax with your face above water and the rest dangling below. If you try this in the Dead Sea much more of you will remain above the surface.

  • i dont think dead people always float. as lots of time when someone is missing they will send diver teams in the water. and i also know live people dont always sink. i can sink or float based on how much air is in my lungs.

  • 1 decade ago

    Actually people sink when dead, and when they start to decay, the gases build up in the cavities, and expand, causing it to rise. Furthermore, when Im in my pool, I dont sink. I float, unless I blow all the air out of my lungs. Otherwise, Im fairly buoyant.

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