So I’ve noticed that I seem to be more bouyant in hot tobs, spas and other similar bathing vessels with jets than I am in a pool or a regular tub. It seems not to matter whetherit’s a commercial tub (at a spa or gym or hotel) or in somebody’s house.
My totally unscientific WAGs are 1) the chemicals used to treat the water act like the salt in (among other places) the Dead Sea and/or 2) the jets cause more air to be dissolved in the water which would have, um, some effect. The flaw in the 1st WAG is that the chemical levels in a home tub are not going to approach those in a commercial tub. The flaw in WAG number two is that it doesn’t really make a whole lot of sense even to me as I formulated it.
So has anyone else noticed a change in bouyancy along these lines? Anyone want to try to explain it to me?
I’ve noticed this as well. I discount effects of chemicals (the main chemical in public hot tubs being chlorine bleach, same as what’s in the swimming pool) because I am not as bouyant in the swimming pool. The only thing I can come up with is that the thousands of tiny bubbles in the wine make me happy.
Whoops, sorry… the thousands of tiny bubbles produced by the jets somehow stick to me and give me lift. I suppose I could prove this by turning off the jets, then all that’s left would be the heat.
There’s at least 3 things in effect as a net force.
The chemicals. Although comparatively small in volume compared to the amount of water, they DO increase density. Density, even a slight change affects bouyancy. The more dense the solution, the more bouyancy on a solute of unchanging density. You can see this first hand. Put one ice cube in a glass of tap-water. Try to keep it from touching the sides, as that will affect results. Add one tablespoon of salt. Notice how the ice cube floats higher, even if just a little? Congratulations, a small upward force. Remember that.
The jets. Kamandi had the right idea. No matter which way the jets are facing, they exert a force. Pointing down, they push off the seats and deflect up. This would obviously be a smaller force than the jets on the seats that do actually point up. Those jets are clearly an upward force. But even jets facing sideways can provide lift. In fact, that is a fundamental property of all fluids. You can ALSO see this first hand. Take a strip of paper, no more than 1 inch wide, but at least 8 inches long. Hold it with both hands such that it is sticking out away from you lengthwise. Hold it up to your mouth and blow underneath it straight out. The paper rises, right? That’s because the air is literally pushing it up. Now, blow over the top of it. It also rises, unless you are doing something wrong. The jets work in a similar fashion. The air bubbles released in the water have a very little effect overall. The jets provide a small upward force. Remember that.
The heat. Heat, as a rule, rises. Well, the heat from the hot-tub, acts as a type of thermal lift. Again, this is a small force, though observable. Smoke signals are an example of this. By holding a blockade over a fire, smoke builds up. Removing the blockade allows the heat to force the smoke upwards. Heat, also increases pressure, which causes tea kettles to whistle. Increased pressure is another small upwards force. Remember that.
So all in all, there are a few small upward forces. Even added together, they only create a slightly less small upward force, not enough to push you out of the water, or even allow neutral bouyancy (neither floating nor sinking). You will always sink in a hot-tub, provided the chemicals are at a safe level, the jets are not supercharged, and the heat will not slow cook you, but you will just sink slower, and thus feel lighter and more bouyant.
If any real physicists find flaws in my deductions and examples, please feel free to clear them up, but applying my two years of Honors Physics, allowed me to answer as I have. Hope this helps, otto.
Though someone might come and contradict me, my knowledge says that the jets should decrease bouyancy as the bubbles in the water would significantly decrease the density of the water. This can be seen in extreme examples at the foot of dams where there is so much air and bubbles in the water that boats will actually sink.
No GulDan, you’re right that thousands of gas bubbles would decrease bouyancy… one of the theories (I don’t know how strong though) behind sinking ships in the bermuda triangle. One a more sure footing is how air lifts work for pumping water… like the ones in your undergravel filter in the aquarium. I beleive the biggest effect in the hot tub would be the water currents created by the jets. The chemicals would probly have a very small effect, though I don’t really know which ones go in hot tub water. The heat shouldn’t do much… the hot water does rise, but sinks again when it cools off at the surface, so it’s more of an overall turnover rather than an net upwards pressure; then again the jets churning the water would over-ride this force anyways - not to mention that the water looses density as it gets hotter.
You can float a log down a river that seems too shallow if the water moves fast with lots of little upwelling currents through it. I’ve stood by a weir where the boiling current was tossing big logs right out of the water, so the jets get my vote. For a simple test of that, just turn the jets off (if you can), and sit in the warm, chemically adjusted water and see if you still feel as bouyant.
Just a minor nitpick, and maybe I’m wrong (IANAPhysicist) it’s not heat itself that rises, hot fluids tend to, so the convection currents caused by rising hot fluids could be part of the phenomenon.
How is pressure going to be a factor in an open vessel like a hot tub?