# buoyant force in liquid vs. air

Discussion in 'MCAT Study Question Q&A' started by SaintJude, Mar 1, 2012.

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1. ### SaintJude

Jan 4, 2012
A cube of wood whose side are each 10 cm weight 16 N in air. When half submerged in an unknown liquid, it weights only 10 N.

Why is the buoyant force the difference between an object's weight in air and that in liquid? (So in this question, Fb=6?)

Last edited: Mar 1, 2012
3. ### chiddler 5+ Year Member

Apr 6, 2010
Because the force depends on the density of the fluid that contains it.

4. ### SaintJude

Jan 4, 2012
Don't understand what you just said.

5. ### milski1K member 5+ Year Member

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Dec 30, 2009
Where the rain grows
By weight in the water they mean the force pulling down the cube. That's its weight minus the buoyant force. In air the weight is just that - the weight.

6. ### SaintJude

Jan 4, 2012
1.) How did you know the "weight in the water" is actually the "force pulling down the cube. " ?

Last edited: Mar 1, 2012
7. ### chiddler 5+ Year Member

Apr 6, 2010
sorry i misunderstood the question.

8. ### milski1K member 5+ Year Member

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Dec 30, 2009
Where the rain grows
What else could it be? Weight is the force that Earth exerts on the object by gravity. So if you want to be very precise, the weight should be the same in both cases. But it's customary to refer as weight to the force that pulls a body down towards the Earth.