Marcellus -
The reason that things float in air is because they're lighter than
air is. It seems a little bit strange to think that air actually weighs
something, but it really does! Air is made up of lots of /really/
little things, like nitrogen and oxygen atoms. These atoms hardly weigh
anything, so gravity doesn't pull on them very hard, but they weigh
enough to be held near the ground. (If you've ever heard of someone
going up very high on a mountain, you've heard that there's not as much
air up there. That's because air does weigh something, and gravity
pulls it down towards the earth.)
How much something weighs for how much space it takes up is what
scientists like to call "density." Something like rocks has a higher
density than something like styrofoam. This is because rocks weigh more
for how much space they take up than styrofoam does. If you were to mix
a bunch of rocks in a box with a bunch of styrofoam peanuts, you'd
notice that the rocks would settle to the bottom, and the styrofoam
would end up on top. This is because things with higher densities
(things that weigh more) get pulled down by gravity harder than things
with lower densities (things that weigh less) do.
It works exactly the same way with air. Some things, like helium
gas, have lower densities than air does. This means that they weigh
less for how much space they take up. So if you have a balloon full of
helium, it will float high up, because it has a lower density than air
does. Things like rocks, on the other hand, have higher densities than
air (they weigh more), so they'll just sit on the ground.
Thanks for the question! Hope this helps!
-Tamara
(published on 10/22/2007)
