Stupid Question

[lablover]

OK the reason I must like water so much is the feeling of weightlessness. (Who wouldnt?) In considering the amount of total weight for tub placement we are asked the dry weight of the tub, the water weight and the weight of the soakers. Tell me how that works when we feel weightless? I think I must have too much time on my hands folks!!

[chem geek]

Though the buoyancy of the water has YOU feel weightless (and it really doesn't feel that way, but more on that later), your weight is nevertheless still part of the total weight in the tub -- basically, you are no different than the water you have displaced in terms of being something with mass that is contributing to the total weight of the tub. The water in the tub doesn't "feel" heavy either -- it "feels" weightless too, in some sense as described below.

If you think about it, you really don't feel weightless anyway when you are floating in water -- you are still being held up by forces, but they are evenly distributed across your body rather than being concentrated in one place, depending on orientation. If you are as upright as you can be, you still feel pressure on your feet (and anything else pointing down as surface area oriented down) while you feel more "weightless" if you lie down because the pressure is then distributed across your entire body similar to lying on a bed. What you don't feel is a "hard" surface -- essentially, the pressure you feel that holds you up moves with you as if you were standing on a floor that moved up when you raise your feet. This allows you to have more freedom of movement which feels as if you don't weigh as much, but if you've even been truly weightless as in free fall (with no wind resistance -- so in an airplane in free fall), then that's a very different distinguishable feeling.

Because you can't feel much of a change in pressure when you move your feet in the water, it "seems" like there isn't anything pushing against you. Your brain is best at distinguishing changes in signals and not so good at constant signals so when you feel constant pressure your brain tends to ignore it -- especially if you move your body and the pressure doesn't change. Your brain associates gravity and feeling weight with being against a hard surface that when you lift your leg up the pressure goes away. Since that doesn't happen in water (the "floor" moves with you), you feel weightless. If you could stand up in a denser gel, then your feet would also feel more weightless than on solid ground since the pressure would be fairly even on your feet and downward-oriented body surfaces, but if you moved your feet up and down, the lower viscosity of the gel would have you feel like it was more "solid" and you had more weight (in both directions since even lifting the leg up would feel resistance), but when you stood still it would feel a lot more weightless than on solid ground.

So the bottom line is that it's a combination of more even pressure on your body plus your brain playing tricks that has you feel somewhat weightless, but in reality your mass contributes to the weight of the tub.

Richard

[lablover]

Thank you Richard for your VERY thorough explanation. Have you considered changing your name to Chem and Physics Geek?