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Q & A: Can something be denser than water and still float?

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Most recent answer: 01/28/2015
Q:
What matter (object) is more dense than water and still floats.
- Kevin (age 43)
logan,ut
A:

Many objects can be made of materials denser than water and still float. For example, ships made out of very dense steel can stay afloat just fine. The reason has to do with the shape of the object, and with how it's displacing water. This is true for materials of any density.

Think about this - say you have a type of rubber which is denser than water. If you throw a ball made out of this material into a pond, it will sink. However, if you use the rubber instead to make a balloon, then inflate the balloon and throw it into the pond, it will float on top. You see, even though it's the same rubber, having all that air in the middle lowers the average density of your object by a lot. The rubber can't just sink by itself now - it brings along this entire cavity of air, which is a no-go.

It's essentially the same with a boat. Because of its curved hollow design, whenever a ship sinks into the water it's not just bringing steel below sea level, but it's also bringing all this air, which ends up making the total ship seem a lot less dense than just the material it's made out of. Interestingly, if we were to replace all the air on Earth with a rare (un-dense) gas like helium, then all ships would ride higher, and if we replace the air with a denser gas, all of our ships would ride a little bit lower.

Matt Z.


(published on 01/28/2015)

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