# Buoyant Force

Simon Bridge
Homework Helper
... only if you answer questions. I gotta turn in for the night. See you tomorrow.

... only if you answer questions. I gotta turn in for the night. See you tomorrow.
Liquid A is less dense, can you continue?

haruspex
Homework Helper
Gold Member
2020 Award
Liquid A is less dense, can you continue?
Right, so based upon your post #20, should it be more stable in the higher density liquid or the lower density one?

(Note: I feel that the question is a bit misleading. Stability of floating objects is rather a tricky subject. A key factor is the second moment of area of the object in the horizontal plane at the surface of the liquid. E.g. consider a rectangular block with a tall narrow pole (mast) going up from the middle. As long as the water line is around the block, the second moment of area is large. With more submerged, only the mast is above the surface, so the second moment of area is much smaller and it heels over.
See https://en.m.wikipedia.org/wiki/Metacentric_height)

Right, so based upon your post #20, should it be more stable in the higher density liquid or the lower density one?

(Note: I feel that the question is a bit misleading. Stability of floating objects is rather a tricky subject. A key factor is the second moment of area of the object in the horizontal plane at the surface of the liquid. E.g. consider a rectangular block with a tall narrow pole (mast) going up from the middle. As long as the water line is around the block, the second moment of area is large. With more submerged, only the mast is above the surface, so the second moment of area is much smaller and it heels over.
See https://en.m.wikipedia.org/wiki/Metacentric_height)
Less density liquid cause the object to become more stable, so the ans for post #14 is the object would be less stable when it's submerged in higher density liquid?

haruspex
Homework Helper
Gold Member
2020 Award
Less density liquid cause the object to become more stable, so the ans for post #14 is the object would be less stable when it's submerged in higher density liquid?
Yes, that would generally be true.

Yes, that would generally be true.
So, the reason that the object is less stable in more dense water is because when the object float higher in liquid, it has more tendency to turn over?

haruspex
Homework Helper
Gold Member
2020 Award
So, the reason that the object is less stable in more dense water is because when the object float higher in liquid, it has more tendency to turn over?
Yes. But as I posted, that is only the general trend. There may be exceptions according to shape.

Yes. But as I posted, that is only the general trend. There may be exceptions according to shape.
Exception according to shape? Can you explain further?

Simon Bridge
Homework Helper
The trick is to realise what gives rise to the stability.
To do that, draw the same shape in two different liquids, as per my example to start with, then do another sketch with them a bit tilted over.
Look to see where the forces act (hint: pressure is perpendicular to the surface, and the pressure at the sides is also important.)

Do you understand where the buoyancy force comes from in the first place?

(I know I said "tomorrow but I have insomnia - and I saw post #25 before post #24 for some reason.)

The trick is to realise what gives rise to the stability.
To do that, draw the same shape in two different liquids, as per my example to start with, then do another sketch with them a bit tilted over.

Look to see where the forces act (hint: pressure is perpendicular to the surface, and the pressure at the sides is also important.)

Do you understand where the buoyancy force comes from in the first place?

(I know I said "tomorrow but I have insomnia - and I saw post #25 before post #24 for some reason.)
Here it is, so what are you trying to say?

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is there anything to do with the metacentric height , buoyant point in this question ?

haruspex