0
$\begingroup$

Understanding Buoyancy

I'm trying to understand buoyancy so I created a question that should help me greatly. To keep this simple my above is a photo is of a 20 ft long tank. Each line represents 1 cubic ft which each holds 62 lb of water. To get specific this full tank would be: 1' x 1' x 20 ' and would hold a total of 1,240 lb of water as each line is 1' x 1' x 1'.

The question is how do I calculate how much weight is needed to sink a specific amount of air? Also how do I calculate how far it would sink considering water pressure increases by depth. I'd like to find the MAXIUMUM amount of air I could sink with the LEAST amount of weight to be at least beyond 11 ft deep.

We can start with this example photo where I've made the section of air 30 % of 1 cubic foot and the other 70% is just of weight. THIS IS A CLOSED CHAMBER which would be:

3.6" x 3.6" x 3.6" = .03 cubic ft = AIR (PINK)

8.4" x 8.4" x 8.4" = .07 cubic ft = WEIGHT = 43.4 lb (GREEN)

In theory this wouldn't be enough air to even lift that amount of weight so I'd assume that It would sink until it remains still due to water pressure at a specific depth.

$\endgroup$
  • $\begingroup$ Please disregard the green details on the drawn photo as it's supposed to say (43.4 lb or 70 % of section). $\endgroup$ – Rip Oct 5 '19 at 16:57
1
$\begingroup$

Buoyancy does not really change with depth (water temperature differences can slightly change it's density). Once a sealed container weighed more than the water it displaced (negative buoyancy), it would begin to sink and go to the bottom. If it weighed less than the amount of water it displaced (positive buoyancy) it will float. So to decide if your container will sink or float, find it's outer volume (that is the amount of water it will displace), find the weight of that volume of water, if the sealed container's total weight is more than the weight of the same volume of water, it will sink to the bottom. If it is lighter than the water it will float. Neutral buoyancy, where it weighs the same as the water, and will neither sink or float, is an unstable condition.

| cite | improve this answer | |
$\endgroup$
  • $\begingroup$ Thank you. Correct me if I'm wrong but I believe you're saying that even a 1,000 lb solid would float or be on the verge of sinking/floating if it was a solid block that's (2' x 2' x 4' ) = 16 cubic feet. Because that exact size would displace roughly 1,000 lb of water. Sound about right? $\endgroup$ – Rip Oct 5 '19 at 18:35
  • $\begingroup$ @Rip: I have not done the math, but it sounds right, 1 cc of water weighs 1 gram, so if a 1 cc container weighed more than 1 gram it will sink, less than 1 gram it will float $\endgroup$ – Adrian Howard Oct 5 '19 at 19:03
  • $\begingroup$ Thank you. May I ask if I dropped a sinkable weight down a lake that's 10,000 ft deep would it hit bottom or will there eventually be a neutral point? $\endgroup$ – Rip Oct 5 '19 at 19:26
  • 1
    $\begingroup$ @Rip It would sink to the bottom, water does not significantly compress in oceans or lakes, so displacement stays the same at different depths $\endgroup$ – Adrian Howard Oct 5 '19 at 20:14
  • $\begingroup$ That raises a question regarding "Pascals Principle" because if so that seems like the "Cartesian diver" would sink to the bottom no matter how deep it is. I may make another question regarding that on this forum after I look up more about submarines. But now that I think of it now I'm talking about an open chamber with air instead of a closed chamber so there will be a difference. $\endgroup$ – Rip Oct 5 '19 at 22:07
1
$\begingroup$

If your sample of air were in a container open at the bottom with extra weights attached, sinking would require that the total weight of the air, container, and weights must exceed the weight of the water displaced by all three. Once submerged, the volume of the air would start to decrease and the system would continue to sink. If the air were in a sturdy closed container, very little compression would occur with a moderate increase in depth.

| cite | improve this answer | |
$\endgroup$
  • $\begingroup$ Thank you. Correct me if I'm wrong here but I believe you're saying that even a 1,000 lb solid would float or be on the verge of sinking/floating (neutral buoyancy) if it was a solid block that's (2' x 2' x 4' ) = 16 cubic feet. Because that exact size would displace roughly 1,000 lb of water. Sound about right? $\endgroup$ – Rip Oct 5 '19 at 18:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.