Buoyancy question BR?

[dannybht]

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If an object floats on the surface of a liquid in such a manner that sixty percent of the object is submerged, which of the following sets of density values accurately describes the liquid and solid?

I don't know understand why in the solution it says "because the solid floats on the liquid, the buoyant force upward is greater than the gravitational force (weight) downward." I thought the buoyancy force balances the weight if the object floats... Help please

 

[ashtonjam]

Maybe it's saying that while the object is completely submerged, the buoyant force is greater than the gravitational force.

EDIT: If the object is less dense than water, my bad.

 

[sps27]

If an object floats on the surface of a liquid in such a manner that sixty percent of the object is submerged, which of the following sets of density values accurately describes the liquid and solid?

I don't know understand why in the solution it says "because the solid floats on the liquid, the buoyant force upward is greater than the gravitational force (weight) downward." I thought the buoyancy force balances the weight if the object floats... Help please

That is probably a misquote or something. Ignore that. For an object that floats, the buoyant force = weight of the fluid displaced.

 

[texan2414]

Maybe it's saying that while the object is completely submerged, the buoyant force is greater than the gravitational force.

This is wrong... When an object is completely submerged, the buoyant force fails to match the object's weight. Why? Because buoyant force can only be exerted on a fraction of an object's volume, not all of it.

 

[ashtonjam]

This is wrong... When an object is completely submerged, the buoyant force fails to match the object's weight.

What I meant was if an object less dense than water is held below the water's surface (completely submerged) and let go, it will rise up to the surface and poke out of the water because the buoyant force exceeds gravity. I'm pretty sure this isn't wrong.

Why? Because buoyant force can only be exerted on a fraction of an object's volume, not all of it.

I keep seeing buoyant force acting on the center of mass of an object in free body diagrams, so this is new.

Actually I think I get what you mean, never mind.

 

[texan2414]

OHhhhh. The object being less dense than the fluid changes everything.

You are absolutely correct in your assessment that Fb = W when its floating. It is a indeed an equilibrium solution. Btw, what is the page no of the problem you're looking at in BK?

 

[dannybht]

It's page 68 of berkeley review chemistry. I think it is probably misquoted as well. After a little bit of digging, I found out tha only if the solid object is pushed down to have it completely submerged it has a buoyant force greater than its own weight. Therefore it rises up when you let go of it and floats again. It's own weight will be balanced with a lesser and equal buoyant force (due to less volume of water displaced). Makes so much sense now...

 

[dannybht]

Sorry AshtonJam didn't fully read your comment. You are correct AshtonJam..thanks!

 

[smiley27]

Yea, I think buoyant force is equal to its weight, because if buoyant force was greater than its weight it would have an acceleration due to the net force, but the object is in equilibrium.