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I was having a discussion with a friend about human tolerances of g-force. He believed that the maximum human tolerance of negative g-force is low, in the order of .5 g. I countered, saying that you're able to stand on your head and do not die. An interesting discussion ensued but there was one thing that we couldn't figure out:

Is there any difference between the negative g-force when you are standing on your head (i.e. you're stationary on the ground, just upside down, experiencing gravity) and negative g-force when you're in motion? For example, like when you are riding a roller coaster that has a bump in the track, and you're forced up from your seat?

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You are correct that standing on your head is the same as a G-force of -1 G. That is what Einsteins equivalence principle tells us. Your friend is correct that tolerance for negative Gs is lower, but it is around -3 G not -.5 G. Tolerance for positive vertical Gs is around 5 Gs without G suits or training. Tolerance for horizontal Gs varies from 12 to 17 Gs depending on whether the acceleration is "eyeballs out" or "eyeballs in".

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'g-force' (or acceleration) is actually a vector quantity. Therefore a negative g-force is identical to a positive g-force in the opposite direction.

This is however a cluttered way of thinking about the situation. I recommend drawing a few free body diagrams to clear up the physical situation.

I am deliberately avoiding giving a more complete answer in lack of a more detailed question.

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Gravity is 1 "G" It normally pulls blood from head to feet. If you turn upside down, you will experience 1 "G" but blood will go from your feet to head.

If you were to tie your feet to something that created centrifugal force and set it for 1 "G" according to your mass, it would be the same as hanging upside down.

Centrifugal force and gravity are expressed by units of acceleration, so they are the same thing except that gravity is constant and accelerates all masses equally. Centrifugal force can be weaker or stronger than gravity.

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Forces are measured in units of Newtons, which is an acceleration times a mass (1 N = 1 kg$\cdot$m/s$^2$); so it's actually incorrect to say that forces "are expressed by units of acceleration." – Kyle Kanos Oct 27 at 16:23

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