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Q & A: Bouyant forces

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Most recent answer: 10/22/2007
Q:
i am wondering if you change the density of water what objects buoyancy will effect it the most
- Connor (age 12)
stockdale christian school, bakersfield ca usa
A:
Hi Connor,

Archimedes' principle states that the bouyant force on an object is equal to the weight of the water the object displaces, or pushes aside. It doesn't matter what the object is made out of, the bouyant force on it only depends on the volume of that part of the object that's underwater.

Some objects don't weigh much but take up lots of space (like styrofoam and beach balls). These float with most of their volume sticking up above the water. Denser objects have larger portions of them under water.

I'm not sure how to assign a numerical value to "buoyancy", but an object that sinks in pure water you might expect to have no "bouyancy" in pure water. If you change the density of the water, say, by adding salt, so the water becomes more dense than the object, the object will then float to the surface. Objects that float anyway in pure water will rise up a little more when salt is added. You can figure out how much the change is by setting the buoyant force (equal to the weight of displaced saltwater) equal to the weight of the object.

Tom

(published on 10/22/2007)

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