this episode is sponsored by Dropbox if
it sometimes feels like the whole world
is pressing in on you well that's
because it is a 1 square meter column of
air above your head stretching from here
to the edge verse atmosphere contains
almost 10 tons of air that's a lot to
carry on your shoulders but if it ever
feels like too much you know it's a
great way to get away from it all
balloons that that's not what I meant
guys not like that like this
balloons are magical birds and planes
stay aloft by using power to keep air
moving over their wings we build blimps
and party balloons with lighter gases
like helium but balloons like those
weigh thousands of pounds and they're
not filled with helium they're filled
with the very same air they're floating
and yet they still go up now that you
mention it why does the atmosphere
itself stay up that column of air above
your head we talked about earlier what's
the same mass as a couple of elephants
but just like elephants air molecules
obey gravity and I don't see any
floating pachyderms around think about
it air is mostly empty space
if I was an oxygen molecule I'd have to
travel the length of a football field on
average before I ran into another one
but occasionally they do collide air
molecules will fall until the collisions
bouncing them up the pressure is
balanced by the weight pushing down on
them up high in the atmosphere there's
not much air on top of us and not much
colliding going on so the pressure is
pretty low but down a little lower we
have a big mass of air above squishing
down so things get more crowded and the
pressure is higher this sets up a
gradient all the way down to earth the
lower we go the air is more crowded it
takes more collisions to balance out all
that squishing even though it's mostly
empty space air can still hold up a
plane I can sit in a chair in the sky
riding on almost nothing and not follow
the ground that is crazy you may have
heard that the Wright brothers are
responsible for humans taking flight but
that's wrong 120 years before them in
1783 the Montgolfier brothers had the
brilliance and kind of dangerous idea to
light a fire underneath a cloth balloon
and humankind took to the skies for the
first
time
heat is what is keeping this balloon up
you probably heard your whole life that
hot air rises but have you ever really
stopped to ask yourself why I mean you
know what we're gonna need a smaller
balloon to figure this one out ah that's
better
this is the ideal gas law tells us
pretty much everything we need to know
about how gases behave pressure is
basically the force over a given area on
a surface whether a balloon inflates
depends on the difference in pressure
inside and out as we heat up air in a
balloon the molecules gain kinetic
energy and start colliding with more
force on the inside surface the pressure
goes up as the balloon expands that
horse is spread across a larger surface
area which lowers the inner pressure
until the pressure on both sides is the
same or until it pops
but this balloon isn't made out of
rubber it's pretty rigid once it fills
up it can't really get bigger but it
does have one thing that the rubber
balloon doesn't a hole in the bottom
you know what the mass of air in the
balloon envelope this is one hundred
five thousand cubic feet
we calculate that the mass on a 5000
pound well I calculated it we first
filled up this balloon on the ground it
held about 3,700 kilograms of air and in
the basket the fuel the people and of
course the balloon itself and that's
another 500 kilograms or so a lot of
mass to lift but then we added something
was we're adding heat to this balloon
those air molecules are moving around
faster and faster with more energy and
they're pushing on the balloon harder
and harder but the volume has remained
the same it hasn't gotten any bigger or
smaller the pressure is pretty much the
same to the balloons not collapsing or
popping so that leaves us with one other
option
there must be less air in that balloon
than when we were down on the ground
hot air has leaked out the bottom we're
still being pulled down by gravity but
the equal volume of cold air around us
weighs more and it's falling down
underneath us balloons don't defy
gravity they float because of gravity
Eureka
this is the principle of buoyancy that
Archimedes noticed when he saw his
bathtub rise it's the reason that ships
float because the weight they hold is
less than the weight they displace and
the water falls down around them we know
that buoyancy is due to gravity because
bubbles rise here on earth and they
don't rise on the space station you said
that playing a balloon was like what
lying on a hot air balloon is the
closest thing to being a cloud that I
can think of we're just floating with
the wind
there's junkless going
flying in hot-air-balloon kinda reminds
me we have something in common with fish
that we live in a fluid
it's just that ours is made of air
instead of water it's kind of like
floating on a bubble that's something
that can lift all our spirits stay
curious
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all yours before the International Space
Station before Skylab before Sputnik
just one special satellite circled the
earth the moon was around before the
first cells divided every life-form
that's ever looked up has seen that same
face since the first humans stood and
walked it's been our big round calendar
in the sky our constant companion so I
guess it must have been pretty
terrifying when people looked up and saw
this