In the last video I gave a
little bit of a hand wavy explanation about why S-waves
don't travel in liquid or air. What I want to do in this video
is give you a little bit more intuitive understanding
of that, and really go down to the molecular level. So let's draw a solid. And it has nice covalent
bonds, strong bonds between the different molecules. And the bonds are drawn
by these lines in between. So if I were to
hit this solid, you know I have this really
small hammer where I just hit at a molecular
level, but if I were to hit these molecules
hard enough so that they move but not so hard enough
that it breaks the bonds, then essentially what
it's going to look like is this kind
of row of molecules is going to move to the left. So you're going to have
that row of molecules moving to the left. And then the row above it
won't fully move to the left just yet, but it will
start to get pulled. So let me just draw
all of the bonds. I'm just drawing all
of the same bonds. Because these are strong bonds
that we have in a solid-- Actually, they could
be ionic bonds as well. Because they are strong bonds
that we have in this solid, they'll essentially be pulled. The top row will be
pulled in the direction of the bottom row. And so they'll start kind
of moving in that direction. And then the bottom row will
essentially recoil back. And then you fast
forward a little bit. And so then the top row
will have moved to the left. And now the bottom
row will start to move back, especially
because, remember, it's bonded to other
things down here. It's bonded to more of
the solid down here. So it would move back. And you can see this
transverse wave, you can see this
S-wave propagating. Essentially right over here
the kind of peak of the S-wave is here. Now it has moved up. Now, let's think about
the exact same situation with the liquids. In liquids you don't have these
strong ionic or covalent bonds between the different molecules. You just have these
weak kind of bonds, usually formed due to polarity. So in a liquid,
water's a good example, you just have these kind
of weaker bonds formed because water is
a polar molecule. So the kind of
half-way polar sides or the half-way positive
sides are somewhat attracted to the
half-way negative sides. So they kind of flow
past each other. But if I were to hit these
water molecules right here with my hammer,
what would happen? Well, they're definitely going
to start moving to the left. And actually, this one's going
to bump into that one, which is going to bump
into that, which is going to bump into that one. They're going to
move to the left. But these molecules aren't
going to move with them. You could view it as
it's going to break that very weak bond
due to polarity. They're going to move
away from each other. Let me draw these top
molecules in green. They're essentially just
going to flow past each other. They're going to
flow past each other. And this guy might have had also
weak bonds with stuff below it, too. I should draw it
as dotted lines. But because of the
impact here, these guys are just going to flow. They're actually going to
compress in this direction. You're going to have a
P-wave, a compression wave, go in this direction, where
this one bumps into that one, and then goes back, and then
this one bumps into that one and goes back, and then this
one bumps into that one. But the bonds aren't
strong enough, and it's even more
the case with air, but the bonds aren't strong
enough for these blue guys to take these green
guys for a ride. And the bonds are
also not strong enough for the adjacent
molecules to kind of help these blue guys to retract
to their original position. So when I talked about the
elasticity in the last video that's what I was talking about. The bonds aren't strong enough
to cause the things that have deformed to kind of
move back to where they were, and also the bonds
aren't strong enough to allow the things
that are deformed to pull other things with it. And so that's why, in general,
S-waves only travel in solid, and they won't travel
in liquid or air.