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Author Topic: Can you experience negative g's?  (Read 9222 times)

Offline erickejah

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Can you experience negative g's?
« on: 02/11/2010 22:24:52 »
Is there anyway in which you can experience negative g's?


 

Offline Bored chemist

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Can you experience negative g's?
« Reply #1 on: 03/11/2010 06:52:24 »
Stand on your head or hang yourself by the ankles.
 

Offline syhprum

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Can you experience negative g's?
« Reply #2 on: 03/11/2010 13:20:26 »
I love these definitive short answers the only problem is that they put an end to all further discussion
 

Offline graham.d

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Can you experience negative g's?
« Reply #3 on: 03/11/2010 13:26:12 »
Clearly, not necessarily :-)
 

Offline imatfaal

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Can you experience negative g's?
« Reply #4 on: 03/11/2010 16:23:03 »
and they are only negative gs in a parochial sense (if standing up is +ve g and turning on a horizontal axis by 180 degrees is -ve g; then what is lying down? zero g?) .  I wonder if the OP was wondering about the fact that gravity is only attractive (barring some weird bits of dark energy) - and asking if there is ever a possibility that we would experience repulsive gravity. 
 

Offline Geezer

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Can you experience negative g's?
« Reply #5 on: 03/11/2010 18:43:51 »
Well, you might say that we experience negative g (or more precisely negative acceleration) every time we put the brakes on in a car, or when we fall out of bed and land on the floor, but it's more of a convention than anything else, because you could just as easily say that we had positive acceleration in a direction opposite to our velocity.

Soooo, (without blowing the dust of any old books or anything) I suppose it's conventional to assign acceleration a positive value if it increases the magnitude of the velocity of an object, and a negative value if it decreases the magnitude of an object's velocity.

Of course, this only works if you assume a particular frame of reference, as in, you assume the the surface of the Earth is stationary etc. Otherwise, things can get considerably more complicated.
« Last Edit: 03/11/2010 18:59:26 by Geezer »
 

Offline Bored chemist

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Can you experience negative g's?
« Reply #6 on: 03/11/2010 18:47:17 »
The definitive short answer here was "Yes", but I was feeling chatty (that's not one of the 7 dwarves).
 

Offline Geezer

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Can you experience negative g's?
« Reply #7 on: 03/11/2010 19:02:26 »

...I was feeling chatty...


So, now we know what chemists do when they are bored  :D
 

Offline Bill S

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Can you experience negative g's?
« Reply #8 on: 03/11/2010 21:06:17 »
Quote from: Geeger
So, now we know what chemists do when they are bored

Hang themselves up by the ankles???
 

Offline Bored chemist

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Can you experience negative g's?
« Reply #9 on: 03/11/2010 22:07:27 »
Nope, we hang up other people by their ankles.
 

Offline syhprum

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Can you experience negative g's?
« Reply #10 on: 03/11/2010 22:14:17 »
Acceleration can only be positive or negative in as much as it a vector quantity like speed as opposed to velocity, is there any way this should be shown in the equations ?.
 

Offline Don_1

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Can you experience negative g's?
« Reply #11 on: 04/11/2010 09:16:11 »
This all seems to have put paid to my answer, which was going to be put a centrifuge into reverse!

I would have thought negative G should be impossible, after all, any force has to exert G force, even if BC has tied my ankles to the belly of a Sopwith Camel, I must experience some G force. Also a real bad headache when landing.
 

Offline imatfaal

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Can you experience negative g's?
« Reply #12 on: 04/11/2010 11:22:44 »
Soooo, (without blowing the dust of any old books or anything) I suppose it's conventional to assign acceleration a positive value if it increases the magnitude of the velocity of an object, and a negative value if it decreases the magnitude of an object's velocity.

Acceleration can only be positive or negative in as much as it a vector quantity like speed as opposed to velocity, is there any way this should be shown in the equations ?.

The first assumption we make is in which direction we define S distance as positive and which as negative - the second is that we do calculus as generally agreed v=dS/dt and a=d2S/dt2. there are no choices to be made about acceleration and velocity only in which direction away from origin we define distance to be positive.  Forces are defined as positive if acting in the same direction as the (unit) vector; in earth's gravity the unit vector is defined conventionally as pointing from centre of earth to centre of object thus f= - (Gm1m2/r2).r_hat and g = - (GMearth/Rearth2). r_hat

Syhprum - you got your speed and vel mixed up (typo I think); speed is the scalar and velocity is the vector. The reason we need r_hats - unit vectors - in the above equations is to make them into vectors with a direction as the mass, radius and G are directionless; and we need the results force and acceleration to be vectors.   
 

Offline graham.d

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Can you experience negative g's?
« Reply #13 on: 04/11/2010 13:04:19 »
I think the original "definitive short answer" was the right one. The question was about "negative g". The normal, everyday interpretation of this is that experienced by fighter pilots when they push the stick forward so that the aircraft is forced into a tight curve with a centre some distance below the pilot's feet. Whereas the more normal maneouvre  of pulling back the stick can cause a huge increase in the g force (to a more positive value) and cause the pilot to black out (through the brain being starved of blood), this negative g maneouvre can cause a "red-out" through high blood pressure in the brain and is potentially much more dangerous. Nonetheless, in WWII the ability to perform such maneouvres gave the German ME109 a significant advantage over the British Spitfire and Hurricane because the lack of fuel injection technology in the British aircraft risked the engine cutting out through fuel starvation.

Anyway, I digress.

It is reasonable to assume that the question was simply about the direction (as represented by the sign) of the apparant gravity normally expressed as numbers of multiples of g (the acceleration due to gravity). In which case, BC's short answer was actually fair enough.
 

Offline syhprum

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Can you experience negative g's?
« Reply #14 on: 04/11/2010 15:31:36 »
Could we visualise true anti G where when we push on a mass it begins to accelerate towards us ? rather like operating the centrifuge the wrong way round !
 

Offline imatfaal

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Can you experience negative g's?
« Reply #15 on: 04/11/2010 16:12:09 »
Oh Yes - BC was right. I was only arguing cos Syhprum said it was going to be a short thread.  And I think we can visualize Anti-G; its like holding a magnet and slowing moving it towards a block of iron. 
 

Offline Geezer

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Can you experience negative g's?
« Reply #16 on: 04/11/2010 17:27:10 »

Actually, the shorter answer is no. (Not only because it's one letter less.)

If standing on our heads makes g negative, then it ought to be zero when we are lying in bed.

But it ain't. The only time it's zero is when you are plunging towards the Earth.
 

Offline Bored chemist

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Can you experience negative g's?
« Reply #17 on: 04/11/2010 18:56:47 »
"Actually, the shorter answer is no. (Not only because it's one letter less.)"
?
 

Offline Geezer

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Can you experience negative g's?
« Reply #18 on: 04/11/2010 19:58:57 »
"Actually, the shorter answer is no. (Not only because it's one letter less.)"
?

Than "yes", of course.
 

Offline imatfaal

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Can you experience negative g's?
« Reply #19 on: 05/11/2010 10:29:03 »
What g do you feel if you are trapped in a thread on a closed loop trajectory like this one?
 

Offline maffsolo

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Can you experience negative g's?
« Reply #20 on: 05/11/2010 12:27:42 »
What g do you feel if you are trapped in a thread on a closed loop trajectory like this one?

Stuck In The Middle With You by Stealers Wheel

What is on each side???  [:o)] :P   |[B)]| ouch not so hard

Yes I am positive, I will experience positive G force inversely, when hung by my feet.

To experience -G force, would you need to have reached an acceleration that surpasses Earth's gravitation acceleration, as plunging towards the earth's surface? 
« Last Edit: 05/11/2010 13:01:09 by maffsolo »
 

Offline JimBob

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Can you experience negative g's?
« Reply #21 on: 05/11/2010 16:55:50 »
Just to add to the confusion -----  can g ever be negative? That to me, uneducated as I am, would mean that two bodied would repel each other, not attract as with a positive g.






Oh cr*p, DUCK!, looks like an argument coming.
 

Offline JP

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Can you experience negative g's?
« Reply #22 on: 05/11/2010 17:06:05 »
The Wikipedia definition (take that for what it's worth) of a g-force is that it's a vector quantity (of acceleration) relative to free-fall.  It has a positive number for its magnitude and it has a direction.  If the object is only moving in one dimension, commonly up and down, then you could assign one way as + and one way as - if you wanted.

I've only ever heard g-force specified as a positive number without a direction, so they're probably just using the magnitude in that case.
 

Offline Geezer

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Can you experience negative g's?
« Reply #23 on: 05/11/2010 18:39:15 »
As you surmised JP, the Wiki definition is a load of bollo slightly suspect.

The term "g" seems to lack a very formal definition, however, I think it basically comes from the force an object has to exert over part of its surface area to remain stationary and intact against the effects of Earth's gravitational field.

In other words, whether you are standing on your feet, lying on your bed, inverted by supporting your weight on the top of your head, or even hanging upside down suspended by your ankles over a cauldron of molten lead, you are experiencing one g. The only way to escape this menace is to be in free fall (including being in orbit around the Earth).

Of course, you can also simulate a similar effect by various centripetal and accelerative (positive and negative) means, and in some cases produce much greater forces on an object than a measly one g. But, try as you might, you can't ever "attract" the body. You can only exert a force against some part of the surface of the body. Therefore, the body only ever "experiences" no g force at all, or a positive g force.
 

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Can you experience negative g's?
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