MCAT Physics G force acceleration

[bobeanie95]

Question: The human body can only withstand a vertical g-force of about 5g before the body has difficulty pumping blood out of the feet and into the brain. Approximately how much upward force could be applied to a 60kg person at sea level before that person risked fainting?

Answer: A person standing motionaless on a flat group experiences a normal force equal to his weight, mg. That corresponds to 1g. Thus the additional upward force from the surface should provide an additional 4g of acceleration, thus Fnet=ma= (60kg)(40)=2400 N.

My point of confusion is that I thought the upward force would be (60kg)(50)= 3000N. Is the question ambiguous since it did not specify whether it asked for the net force on the individual (book's answer) as opposed to the total force on the person by the ground (my answer).

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[FutureD0C17]

It says the "body can only withstand 5 vertical g's" and what upward force could be "applied" before hitting this threshold. So, we want to apply a force that will result in 5 g's, which means net since the normal is a given and not applied.

 

[bobeanie95]

Makes sense, thank you!