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Pressure exerted on fluids

  1. Feb 3, 2010 #1

    I'm trying to investigate how the force exerted onto a human body is affected by the presence of a surrounding liquid. Since the liquid cant compress the pressure is distributed in all directions. If a person is accelerated with this liquid, he/she + the mass of liquid will feel a force (F= ma).

    If we consider the person alone he/she will feel a certain force due to acceleration, adding in the mass of the liquid will increase this force, so how does the liquid the person is immersed in, make a different to the pressure received by that person?
  2. jcsd
  3. Feb 3, 2010 #2


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    G suits are a good example of this happening. Left to itself, a body in the seat of an aircraft in a tight curve will flatten into the seat and the blood will flow down into the leg and lower abdomen veins , starving your brain of blood and oxygen. The G suit is a strong envelope which can't distort. There is a small amount of fluid in the suit (between two shells so you don't get wet). When you go into the curve, the pressure of the liquid in the legs of the suit and lower parts of the suit balances the hydrostatic pressure in your veins so blood doesn't flow into them and away from your brain. If you were immersed in a tank of liquid, instead of a G suit, the same thing would occur.

    The brain gets similar protection against knocks as it floats in a fluid inside your rigid skull.
  4. Feb 3, 2010 #3
    Doesn't the fact that there is now more mass on the person mean there is a bigger force (assuming same acceleration of aircraft) directed on to him/her, hence how does pressure differ? Im not sure im being very clear.
  5. Feb 7, 2010 #4
  6. Feb 7, 2010 #5


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    The questions don't make a whole lot of sense to me.
  7. Feb 7, 2010 #6

    Doug Huffman

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    Imagine G. H. Hardy's patience with S. Ramanujan who was brilliant but un-schooled.

    Last edited: Feb 7, 2010
  8. Feb 8, 2010 #7
    Yes I had already read this. I guess the question is simply if the body is mainly composed of water then why cant it distribute the forces of high acceleration as omnidirectional pressures just as water does?
  9. Feb 8, 2010 #8
    But how can a liquid compress pressure?
    It will increase. The liquid has to get normal reaction from somewhere and it will be the dude inside the fluid.

    And it's static pressure which distributes itself, here it's dynamic pressure...so the force won't distribute.
  10. Feb 8, 2010 #9


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    With lungs full of air, the thorax can collapse under the hydrostatic pressure until the pressure equalises. Blood (and even soft tissue) from the head - or whatever other bits were 'above' the lungs will tend to flow downwards - pushing the lower bits of the lung upwards.
    There is no difference between static and this "dynamic" pressure you mention because the timescale is much longer than any other time constants in the system.
    I would imagine that, under moderate g forces, an air pressure regulator system could 'blow' air into the lungs to maintain their volume. This is, in effect, what SCUBA valves do and they can operate under quite extreme conditions. You would probably need a tube to bypass the epiglottis, though, to avoid all the problems which divers can encounter when they don't breathe out whilst coming up.
    Last edited: Feb 8, 2010
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