What Determines the Upward Movement of a 1m^3 Balloon in Water?

[Doc Al]

“Because of buoyancy the object feels lighter but it does not become lighter”? It DOES become lighter.

Don't confuse actual or 'true' weight--the force of gravity on an object--with the object's apparent weight--a measure of the force needed to support the object. Obviously dunking an object into a barrel of water won't change the Earth's gravitational pull on it. What does change is the amount of force required to support it--it feels lighter. If the buoyant force happened to equal the true weight of the object, the object would feel 'weightless'--it would just float without needed any support.

 

[fizzickschick]

OMG, entirely overcomplicated. The assumption that there is no pressure difference at the top and bottom of the object didn't really get addressed. There's always a pressure difference, unless you have a dimensionless object. The difference in pressure is what causes the buoyant force, we all agree. So if the object does not rise, does this mean there is no pressure difference, or is there somthing else going on? Namely, the weight of the object! A generic metal hemisphere could rise or sink, depending on the overall density, but the one stated in the original question is neutrally buoyant. Was the original contributor to this thread confused about how this is possible? A simple thought experiment would do the trick. Imagine a block of lead sunk at the bottom of a container. Then the buoyant force is less than the weight of the block. Now remove a piece of lead from the inside of the block and replace it with air. If the air bubble is small enough, the block will still be sunk. But if enough lead is replaced with air, eventually the weight of the block + air will be less than the buoyant force (weight of fluid displaced, rho v g). There. Why is this not simple to explain?

 

[fizzickschick]

Also, I think luckis has a few questions that could be answered by a referal to Newton's 2nd Law and suggesting he draw an FBD.

 

[JaredJames]

OMG, entirely overcomplicated. The assumption that there is no pressure difference at the top and bottom of the object didn't really get addressed. There's always a pressure difference, unless you have a dimensionless object. The difference in pressure is what causes the buoyant force, we all agree. So if the object does not rise, does this mean there is no pressure difference, or is there something else going on? Namely, the weight of the object!

I've discussed the weight and buoyant force over and over and the OP is getting confused with actual weight and buoyant weight (the weight the object feels).

A generic metal hemisphere could rise or sink, depending on the overall density, but the one stated in the original question is neutrally buoyant.

Could you show me where it states neutrally buoyant? His OP regarding the hemispheres tells us:

a metal-coated hollow hemisphere with almost the density of the air (including the metal coat), it raises or not?

If the overall density is virtually that of air, it will definitely rise (it has been pointed out to me that with an air tight seal (perfect seal) between the perfect hemisphere and the bottom of a tank there would only be pressure acting downwards and so the hemisphere would sit on the bottom despite density. How you would get a perfect seal with a metal hemisphere I don't know and would be interested in evidence showing such an example in real life.)

Was the original contributor to this thread confused about how this is possible? A simple thought experiment would do the trick. Imagine a block of lead sunk at the bottom of a container. Then the buoyant force is less than the weight of the block. Now remove a piece of lead from the inside of the block and replace it with air. If the air bubble is small enough, the block will still be sunk. But if enough lead is replaced with air, eventually the weight of the block + air will be less than the buoyant force (weight of fluid displaced, rho v g). There. Why is this not simple to explain?

As I explained, if the overall density of an object is less than that of the displaced water, it floats (as per his example). If it is more than the displaced water, it sinks. If it is equal, it is neutrally buoyant. Nothing difficult there.

The problems arise in the previous posts when the response from the OP were constantly contradicting over numerous lines. He would make a statement in line one and then contradict it a few lines later. Not easy to work with.

I'd also point out that certain posts here have been edited after my responses given and so now don't fit well with what I wrote in response.

Also, I think luckis has a few questions that could be answered by a referal to Newton's 2nd Law and suggesting he draw an FBD.

I refer you to post number 2.