Archimedes Principle

[MedPR]

I think I understand how this works, but I want to be sure I'm explaining it to myself correctly.

Archimedes principle basically describes the normal force exerted on a solid by a fluid...?

From mcat-review dot org.

Archimedes' principle: buoyant force on an object = weight of the fluid displaced by the object.
FB = weightdisplaced = mdisplacedg =ρfluidVsubmergedg
The volume of an object that is submerged = the volume of fluid displaced by the object.
Things float when FB = Weight.
Things will rise upward when FB > Weight.
Things will sink when FB < Weight.

I'm having trouble with the wording of this definition. If you put an object with density 0.5g/cm^3, it will "rise upward" in water since the density of water at is 1gm/cm^3?

What is the difference between "rise upward" and "float"?

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[chiddler]

When bouyant force = weight, then it will sit on top of water in equilibrium not really moving.

When bouyant force exceeds weight, then it will actually move upwards until bouyant force = weight where it will then not move and float.

 

[mcloaf]

The difference is where you're starting in your fluid. If you begin submerged, then the object will rise to the top, where it will float. If you start on top of the fluid it will simply stay there. The percentage of the object that is submerged when it floats is proportional to the density of that object relative to the surrounding fluid.

 

[milski]

I think I understand how this works, but I want to be sure I'm explaining it to myself correctly.

Archimedes principle basically describes the normal force exerted on a solid by a fluid...?

From mcat-review dot org.



I'm having trouble with the wording of this definition. If you put an object with density 0.5g/cm^3, it will "rise upward" in water since the density of water at is 1gm/cm^3?

What is the difference between "rise upward" and "float"?

If the body is fully submerged, it will displace more fluid than its weight and that will result in a force pushing the body to rise. As the body moves up, more and more of it will go out of the water and the force pushing up will decrease. At some point it will be equal to the weight and the body will stop rising. You can think of floating as the equilibrium which is established when the weight is the same as the force pushing it up.

 

[MedPR]

So when Fb = mg, the object is partially submerged, but when Fb>mg, none of the object is submerged?

 

[chiddler]

So when Fb = mg, the object is partially submerged, but when Fb>mg, none of the object is submerged?

If Fb > mg, then the object is submerged enough to cause buoyant force to lift it upwards.

Consider a slab of ice. If you submerge it all the way under water, then it will rise. Why? Because buoyant force exceeds the weight of the ice. What happens when it reaches the top? Like you wrote, object is partially submerged. So when Fb > mg, then it means that it displaces enough fluid to cause the buoyant force to exceed its own weight.

 

[milski]

What chiddler said. Some additional clarifications:

None of the object submerged -> Fb=0, Fb<mg
Partially submerged, Fb increases as it submerges more
- it may eventually become Fb=mg - then the object will float, submerged that much
- if the object is too dense Fb will still be Fb<mg even when fully submerged - it will sink
Fully submerged - depending on how dense the object is, it's possible Fb <=> mg. Correspondingly, it will sink, stay at any depth with no other forces, or emerge to equalize Fb and mg.