## Engineering Question #2166

Linda O\'Neill, a 40 year old female from Dartmouth asks on July 7, 2004,

How can you define the pressure of 5 g. on a person? This would be done with the fingers. I am trying to figure out how to equate 5 g into pressure with the finger tips or if this is possible.

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### John Jones answered on July 7, 2004

A 'g' is the force of gravity on Earth, which is not a pressure but an acceleration. To make a person experience an acceleration of 5g by pushing on them with your fingertips, you would need to push sufficiently hard that after a second, they are moving at a speed of 50 metres a second, or about 100 mph. The force you would need to exert to do this would the same as the force you would exert to lift five people slowly into the air -- one g holds one person down on the Earth, so 5g would be enough to keep 5 down -- and this is probably impossible unless you're extremely strong.

You would also need to make sure that the force you exert is evenly distributed over the person's back or front; if you concentrate the force on your fingertips, your fingers will probably go right into the person's body, which would be uncomfortable.

A person might experience an acceleration of 5g when being launched in a rocket, for example, in which case they would be lying on a soft couch. The acceleration is caused by the force exerted by the couch on the person's back. For the most comfortable ride, the force will be distributed evenly over the person's back, creating a pressure of about 5,000 Pascals at each point. Now, you can easily exert a pressure of 5,000 Pascals with your fingertips, but the pressure will only be acting on the area covered by your fingertips. This will not cause an acceleration of 5g, and will not feel much like being launched in a rocket. The closest you can get to the sensation of being launched is probably to have the person lay face-down on a clean, level surface, place a board on their back, and have four friends join you in standing on the board.

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