Question about Bouyancy

[theA1doctor]

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Just need to clarify.

If I have an object that is say 4000 kg/m3 and I put it in water.

All this piece of information tells me that the object will be completely submerged.

Does density information give me any information about how far it is submerged in the water? How can I figure out how far does it sink? Is the only only way to do that through a force diagram? and lastly under what circumstances will an object just hand around in the middle of the tank i.e fully submerged but not touch the floor of the tank?

Thanks!

 

[AwayFromReality]

Yes if you know both the density of the object and the density of the fluid you can find out how much the object is submerged. If you actually do the entire force diagram you will get the following formula. (This formula only applies to objects that have density less than that of the fluid).

%submerged = Density of the object / Density of the fluid.

Memorizing this formula is a good idea but you should absolutely know how to get to this formula using the force diagram because the MCAT goes out of its way to make sure you have a solid understanding about how formulas are derived.

 

[Cawolf]

I agree with @AwayFromReality and the formula he wrote is perfect.

If you derive out the formula you will see that any object that is more dense than it's fluid medium, will sink - how far? All the way. How fast? This requires forces.

The forces on an object in a fluid medium (ignoring drag) are F = mg[(rho fluid/rho object) - 1]. Easier to derive than memorize for sure.

 

[theA1doctor]

I agree with @AwayFromReality and the formula he wrote is perfect.

If you derive out the formula you will see that any object that is more dense than it's fluid medium, will sink - how far? All the way. How fast? This requires forces.

The forces on an object in a fluid medium (ignoring drag) are F = mg[(rho fluid/rho object) - 1]. Easier to derive than memorize for sure.

This was my question

So you are saying that if the density of the object is greater than the liquid, then it will sink all the way? Is there ever a scenario where the object doesn't sink all the way (ignoring drag)?

 

[Cawolf]

Not that I know of.

If the object is denser than the fluid the net force is in the direction of gravity, and it will sink until it meets opposition (bottom) - even if slowly.

 

[theA1doctor]

Thank you sooo much guys!