Questions about water, buoyancy, weight, engineering

[Zelly]

More random thoughts I have while brushing my teeth:

If I have a tub of water and then add a buoyant object (let's say 10lbs), how much weight/stress is being placed upon the structure supporting the tub (aka floorbloards)? 

Is it the overall tub 10 lbs heavier or does the object's buoyancy reduce it somehow?

And in terms of stress on the structure, would a 10 lb sinking object place more stress than a buoyant object considering it would create a point of physical contact?

I'm curious about pools they've built recently in fancy hotels where you're hundreds of feet in the air and it has a glass bottom.  

[Marukka]

3 hours ago, Zelly said:

More random thoughts I have while brushing my teeth:

If I have a tub of water and then add a buoyant object (let's say 10lbs), how much weight/stress is being placed upon the structure supporting the tub (aka floorbloards)? 

10lbs, weight can't vanish.

3 hours ago, Zelly said:

Is it the overall tub 10 lbs heavier or does the object's buoyancy reduce it somehow?

Just like if you would add 10lbs of water. 

3 hours ago, Zelly said:

And in terms of stress on the structure, would a 10 lb sinking object place more stress than a buoyant object considering it would create a point of physical contact?

Yes, if the object is not resting on the pools floor, the water would distribute the additional weight and preassure equally across the whole pool. If it rested at the pools floor, the additional preassure would only affect the area it rests upon. But, this additional preassure would not exactly equal the one that would be caused by the objects weight, but would be lessend by the weight (and therefore preassure) of the water that the object would have pushed away. The more dense the object is, the higher the stress on the pool. The average density of a human is very close to the density of water, and I would guess, that these pools are built with a lot of safety strengh, even if someone would throw some lead weights in them. I still would not like to swim there... 

[Quantus]

A Useful tool they taught us for this kind of thing is to draw a "Control Boundary" around any piece of the system, and you can treat it as a single object (more or less).  So in this case you can take everything on top of the structure as a whole, and if you add 10 lbs of Anything to it, it will add 10 lbs to the weight being supported by the structure.  It's boyancy within the water really just determines how it settles inside the container, and at most adding it will add to the pressure on the sides of the vessel since they are now supporting water+object.

Now, technically, all objects also do have Buoyancy in the any fluid including the Air itself, but that actual value is so miniscule we can and do ignore it in all but the most precise cases.  It's like how technically any two objects have a gravitational force between them, but the only ones that are big enough to factor into most anything practical are the astronomically large ones.