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Name: Adam
Status: Other
Grade: Other
Location: PA
Country: United States
Date: April 2006


Question:
I am a fuels troop is USAF. There is a rumor that I am trying to squash, that it would be impossible to swim in jet fuel (JP-8) because it is less dense than water and your body is comprised partly of water. I have countered that your density is not what makes you swim, it is the upward thrust generated by the motions you make. The density of JP-8 jet fuel is 6.7 lb/gallon @ 60 degrees F, and I have found the density of water is around 8.35 lb/gallon at the same temperature. I have contended that this difference is not enough to render the thrust generated by a swimmer ineffective.



Replies:
I do not believe myself fully qualified to support or debunk you position, but I would like to add a couple of details and ask a question or two.

How viscous is JP-8? If it is very thick, swimming would be relativly easy, if it is very thin, well, it will be much harder to effectivly apply force to the fluid. Being less dense than water would definately (by itself, ignoring viscosity) make swimming more difficult, though probrably not impossible. An interesting paradox arises though as anyone who is a good enough swimmer to stay afloat in jet fuel probrably is not the kind of person to risk their health swimming in jet fuel.

Ryan Belscamper


The principle is this a body is buoyed up by a force equal to the volume of water (or JP-8) displaced by the body. So shape plays a big factor. Obviously, ships are more dense than water but still float. Icebergs float, but only about 10 to 12 percent of their mass is above water. Remember the ill fated ship Titanic. Submarines and other submersibles float or sink depending upon how much air or water are in their ballast tanks. I do not know the density of JP-8 fuel but it is probably about 7.5 to 8.0 lb/gallon, so whether a person could displace enough JP-8 to remain afloat probably depends upon his/her body volume and how they are oriented. It is also the case that by "dog paddling" so as to give the body an upward thrust that a heavier body will float on a lower density fluid, but there is a limit to how much upward thrust one can generate. Remember, of course, that helicopters "float" in the atmosphere by thrusting massive amounts of air downwards.

Vince Calder


Dear Adam,

I am sorry to be so late, but I think I can add some insight to the archived answers to this interesting question.

Since most people have roughly neutral buoyancy (I can float, but can sink by exhaling deeply), their average density is about that of water. A 200 lb person would thus displace about 24 gallons of water (water has a density of 8.35 lb/gal). This person would then displace 24 gal of jet fuel which would generate a buoyant force of about 160 lb (assuming his volume is unchanged).

This means that he would have to exert an upward force of 40 lb with his arms and legs to keep from sinking, I should think most people would find this VERY difficult and would soon sink.

Incidentally, a ship must have an average density less than that of water; otherwise it would sink.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University


Dear Adam,

I believe you would find it very difficult, if not impossible, to swim in jet fuel for more than a very short time, even ignoring problems your body may have coping with the fumes and itching. You are correct, of course, that you can use arm movements to try support yourself in jet fuel, but I think you would find that the 40 lb of vertical thrust you need would be difficult to maintain.

I get the 40 lb by first assuming the human body has the same average density as fresh water (62.5 lb/ft^3 = 8.35 lb/gal). A 200 lb man would thus weigh as much as 24.0 gallons of water and so would just float when immersed in water. However, 24 gallons of JP-8 weighs only about 160 lb (24 gal times 6.7 lb/gal), so this 200 lb man immersed in JP-8 would feel a buoyant force of only 160 lb, and needs an additional 40 bl of vertical force to keep from sinking.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University



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