The answer is one word: buoyancy. What does
that mean? Well, let's think of an easy example:
a piece of wood floating on water. What makes the
piece of wood float? It turns out that when you
put a piece of wood in water, some of the water
has to move out of the way to make room for the
wood; this is called displacement. When this
happens, the water responds by pushing on the
thing causing it to displace (this is because
water has pressure). This upwards push is called
the buoyant force. If the object displacing the
water is light enough (the proper word to use in
physics is "density"), the water can push up on it
hard enough to balance the force of gravity
pulling it down, and so it floats. Since wood is
less dense than water, it floats!
So, how is this relevant to a hydrogen balloon?
Well, it's exactly the same thing. You can think
of the atmosphere (the air around us) as the
water, and the balloon as the piece of wood. The
balloon has to displace some air, so in response,
the air pushes up on it. Now, most things are
much denser than air, so we barely notice this
effect. But hydrogen is less dense than air, so
the air pushes up on the balloon harder than
gravity is pulling it down, and the balloon tries
to move up! So just like wood float on water,
hydrogen balloons float on air.
Objects float when immersed in fluids due to
differences in their densities. Less dense objects
float in more dense materials. Density is a
measure of how much substance per unit volume an
object has. For example, styrofoam (think about
how porous it is!) is less dense than water, so it
floats on water.
So why do hydrogen balloons float in air? Air
is a fluid that is comprised of about 79%
nitrogen, 20% oxygen, and trace amounts of other
gases. Since most of air is made of nitrogen and
oxygen, which have heavier molecular weights than
hydrogen, air is denser than the hydrogen-filled
balloon (the atmosphere is denser than the
hydrogen-filled balloon). This results in
the balloon floating in the air!
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