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B G-Force Displacement -- Is it possible?

  1. Dec 4, 2016 #1
    Hi Guys

    I was wondering if the effects of G-Forces can be displaced?? e.g if a body or object is placed in water (more like encapsulated in water) & is then subjected to tremendous acceleration very quickly, would there still be a G-Force effect on the object inside the water?
     
  2. jcsd
  3. Dec 4, 2016 #2

    BvU

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    Hello Paul, :welcome:

    Remember Archimedes' law ?
    You would have a G-force effect of the effective g minus one and a pressure effect of the effective g. I don't think I like the idea of trying it out in a centrifuge at all.
     
  4. Dec 4, 2016 #3
    Hi 'ByU'

    Thanks for the reply. Im not following you or perhaps just not understanding the terms you used?? Would you be so kind as to elaborate on "You would have a G-force effect of the effective g minus one"

    & is the pressure you referring to, the pressure that would be created when the fluid rushes to the back (back being relative to the direction the object is moving in) of the capsule"??
     
  5. Dec 4, 2016 #4

    BvU

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    I'll try to make myself a bit clearer (actually: fix a wrong answer...o:)) :

    Archimedes discovered he felt more or less weightless in the bath: the g-force is one g less than outside the water. In an environment where a 100 liter person with a mass of 100 kg pushes down with a force of mg = 100 g Newton (##F = ma## and the water pushes up with the with a force of 0.1m3 times 1000 kg/m3 times g Newton (## F = ma = \rho V g ##) the two forces add up to an almost zero net force.

    Now in the centrifuge: the acceleration is no longer 9.81 m/s2 but for instance 5 times as much. So let's pick something, say ## a = 5 g = 49## m/s2. You see both forces multiply by five, so the person experiences 5g from the water alone. I.e. the G-force effect is compensated for and my answer ( 'G-force effect of the effective g minus one' ) was wrong. It will be a bit like being 50 m below see level, but the difference in pressure front and back will be five times greater to compensate ## 5 mg ## instead of ##mg##. Uncomfortable.

    Fighter pilots wear g-suits to fight off unconsciousness; the early ones used water.
     
  6. Dec 4, 2016 #5
    Hi ByU

    So basically, water does not displace the G-Force effect & actually to an extent increases it??

    Also, if at all, is it even possible to displace the G-Force effect in any way??
     
  7. Dec 4, 2016 #6

    BvU

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    Somehow you have to accelerate the object in question. Newton's law ##F=ma## is pretty unavoidable. The best you can do is divide the required force optimally over the body.

    I don't think being submerged in water increases the G-force effect per se.
     
  8. Dec 4, 2016 #7

    jbriggs444

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    The advantage of being placed in water is that the external force that is accelerating you is spread evenly/proportionately all around your body. More supporting force where you are thicker and less where you are thinner. This reduces the stress that the body would otherwise need to withstand so as not to bend, break, bruise, squish, stretch or twist. If the body is not evenly supported, such things happen.

    The ideal would be for the body to be totally submerged in a fluid that is exactly as dense as the body. This minimizes stress, but does not eliminate it. The problem is that your body is not uniformly dense. Your bonds are more dense than your flesh and the air in your lungs is much less dense. As g forces increase, stresses due to density imbalances increase. One approach that could improve things quite a bit would be to replace the air in your lungs with a fluid with a density similar to water and a good ability to contain oxygen and carbon dioxide (e.g. a PFC).

    Replacing the air in the lungs also deals with differences in compressibility of air versus flesh and bone.
     
    Last edited: Dec 4, 2016
  9. Dec 4, 2016 #8
    Hi jbriggs444 - thanks for your reply..

    Another simple question... when in space do astronauts have to deal with G-Force??
     
  10. Dec 4, 2016 #9

    Ibix

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    If their rocket is on, yes. Otherwise they're in free fall.
     
  11. Dec 4, 2016 #10
    At the present time the only astronauts are those on the ISS, and in their case they have to deal with a zero g environment.
     
  12. Dec 4, 2016 #11
    Hi Ibix

    Im a little bit lost here?? If there is no gravity in space, how comes there's G-Force, isn't G-Force a result of gravity?
     
  13. Dec 4, 2016 #12

    Ibix

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    Depends what you mean.

    It isn't true to say there's no gravity in space. How do you think the Moon stays in orbit? However, if you have your engines off you are correct that you will float around feeling no force due to gravity because you are moving freely along with your spaceship. So you are (nearly) weightless in space.

    However, "g-force" in the sense felt by a pilot looping-the-loop has nothing to do with gravity and everything to do with the force being exerted by the engines and wings of the aircraft. If you go round a corner in a car you'll find yourself pressed towards the wall; if you do it in a fast racing car you'll need to do strength training on your neck before you can keep your head upright. If you press the accelerator you'll feel yourself pushed back into your seat. All of this is classified as "g-force".

    Similarly, the effect of a rocket engine firing is to make it feel like the back wall of the rocket has become the floor. Everything floating around in the cabin will fall towards it (or, put another way, the back wall will come forward and hit anything floating in the cabin) and, if the rocket is accelerating at 9.81ms-2 (1g) it will feel just like walking around in a room on Earth. No, it isn't gravity. But (give or take engine vibration) it's indistinguishable from gravity. So we call it g-force. And you have to deal with it whenever you fire your rockets.
     
  14. Dec 4, 2016 #13
    Cool understood

    So what would the effect be on you if the engines are fired but you are physically tied to the ship e.g perhaps like being strapped into a harness but still floating in weightless environment, with your arms and legs strapped to the ship??
     
  15. Dec 4, 2016 #14

    Ibix

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    When the engines arein you aren't weightless. It would just feel like being strapped down on Earth - fine, as long as it isn't the ceiling you're strapped to. Of course, the acceleration might be less than 1g, in which case you'll feel light, or more than 1g, in which case you'll feel heavy.

    Best example I can think of is a lift. You get in at the bottom and the doors close. The lift starts and you feel heavy until it reaches its maximum speed. Then you feel normal until the lift slows down, when you feel light until it comes to a stop.

    Instead of the lift cables you have a rocket motor.
     
  16. Dec 4, 2016 #15
    So, if the speed (acceleration) is maybe 4 or 5 times '1g' one feels heavy if you would for example try to lift up your arm??
     
  17. Dec 4, 2016 #16

    BvU

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    Yes. Acceleration, not speed.
     
  18. Dec 5, 2016 #17

    A.T.

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    No. On the ISS there still have most of Earth’s gravity but feel zero g.
     
  19. Dec 5, 2016 #18
    Hhmnm interesting.
     
  20. Dec 5, 2016 #19

    BvU

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    Do you understand why they feel 0 g ?
     
  21. Dec 5, 2016 #20
    Not really, any offer from my side would be a guess..

    Logic suggest's they're too far away to really feel gravity even though its still there & maybe that's because of their total mass, bearing in mind the moon is still attracted to Earth, proving the existence of Earths gravity out there..
     
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