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Q & A: Buoyancy

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Most recent answer: 06/04/2018
Q:
Can you tell me some information on buoyancy? (for water)
- Louise (age 12)
Shanghai, China
A:

Louise -

Buoyancy is basically a fancy word for how different things float. Whether you’re talking about water, air, or anything else, buoyancy happens because different things have different "densities". Something’s density is basically how much it weighs for how much space it takes up (mass / volume) - you can think of this as how tightly packed it is. If you mix something with a pretty high density (tightly packed) with something with a pretty low density (loosely packed), the stuff that’s more dense will settle to the bottom.

This is basically what happens with buoyancy. If something is more buoyant, then it is more likely to float to the top - i.e. it has lower density. So if you put something in water, all you have to do is think about how dense it is to figure out if it will sink or float. If something is more dense than water (like a heavy rock), then it will sink. But if it weighs less for how much space it takes up (like a cork), then it will float.

But water doesn’t always have the same density either. Think about a tall column of water. The water at the top doesn’t have anything above it, but the water at the very bottom has all the water above it pushing it down and squishing it together. So the water at the bottom is actually a tiny bit more dense than the water at the top. This means that if you have something that’s more dense than the water at the top but less dense than the water at the bottom, then it could "float" halfway in between - it’s more buoyant than the water at the bottom and less buoyant than the water at the top. (See below for more clarification.)

-Tamara


(published on 10/22/2007)

Follow-Up #1: buoyancy

Q:
You have explained bouyancy by saying if the object is less dense it floats, but why? Could you actually explain the forces that make this happen? ie I have heard it is to do with the pressure difference between the top and bottom of the object but then how does volume or density of the object apply. Would be great if you could clear this up for me...
- Matt (age 18)
England
A:
Think of any little layer of the water. It has some weight. It would fall unless there were some force pushing it up, canceling the gravitational weight force. That means that the water beneath it must be pushing up a little more than the water above it is pushing down. In other words, the deeper you go, the higher the pressure must get.

In practice, water, like many other fluids, is only very slightly compressible. The deeper water is at higher pressure, but that corresponds to only a small increase in its density.

Anyway, if you place an object in water that weighs the same amount as the water it displaces, the pressure and weight forces on it will just cancel, the same as they would for the water it displaced. If it's heavier than the water it displaces, then the weight force wins and it will sink. If it's lighter, the pressure difference wins and it floats up to the surface.

So whether an object floats in water depends on whether it weighs more or less than the same volume of water. Weight density means weight per volume, so this is the same as asking whether it is more or less dense than water.

Mike W.

(published on 04/08/2010)

Follow-Up #2: Noah meets Archimedes

Q:
WHY WAS IT THAT NOAH,S ARK LADEN WITH PEOPLE AND ANIMALS, SO HEAVY,DID NOT SINK......I THOUGHT OF THE SAME QUESTION WHEN I SAILED ON MANY CRUISE SHIPS AS THE JEWISH CHAPLAIN....I HAVE HEARD ABOUT THE TERM "DISPLACEMENT" BUT I REALLY DO NOT UNDERSTAND WHAT IT MEANS AND HOW IT WORKS
- rabbi bob miller (age +70)
dedham ma. usa
A:
Hey Rebbe Bob- Ellen told me to anticipate this question. I've marked it as a follow-up because we've partly addressed it before.

There are several equally good ways to understand why a boat floats. The key thing in any practical cargo boat is that there is a lot of air inside. That means that that density of the cargo, counting all that nearly empty space, is a lot lower than water. The water that occupies as much space as the boat weighs more than the boat.

You know that heavy things tend to fall. As the boat sinks into the water, the displaced water (water that had been where the boat now is) has nowhere to go but up.  So with the water going up and the boat going down, is that overall stuff falling or rising? At first, as the boat just starts to go into the water, not much water gets pushed up, so there's net falling. As more water gets pushed up, the falling boat and the rising water balance, so the boat floats. Of course if the boat is heavier than the water it displaces, it just keeps falling.

Now as for whether it's practical to get reproducing populations of 1.7 million or more species (mostly insects) on a boat and keep them alive for a while, we'll save that argument for another day.

Mike W.


(published on 07/06/2011)

Follow-Up #3: buoyancy vs. depth

Q:
I don't think the last paragraph of the answer is correct. "This means that if you have something that's more dense than the water at the top but less dense than the water at the bottom, then it could "float" halfway in between - it's more buoyant than the water at the bottom and less buoyant than the water at the top."The buoyancy force is independent of the depth of the object in the fluid. F_buoyancy = (P_bottom-P_top)*A if we are talking about a simple cylinder shape. P = rho*g*h, and because the top and bottom of the object is always constant(the height of the object). Working out the math, you get F_buoyancy=rho*g*Volume_fluid. The only way the object would "float" half way is if the weight_object=F_buoyancy and you place the object at that location.
- Alan
Seattle, WA
A:

You're right that the density "rho" of pure water increases very little with depth. Therefore it would take a very delicately adjusted float density to be bouyant at large depth and not buoyant at a shallower depth. In salty water, you can ften find situations with denser salt water at the bottom and fresher, less dense, water near the surface. In those cases you can get objects that float part-way up where their density matches that of the water.

Mike W.


(published on 06/04/2018)

Follow-up on this answer.