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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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