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Why S-Waves Only Travel in Solids

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Why S-Waves Only Travel in Solids

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In the last video I gave a little bit of a
hand wavey 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, it'd 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
with this really small hammer,
just hit it at a molecular level
If I were to hit these molecules hard enough
that they move but not so hard that it breaks the bonds
then what it's going to look like is
this row of molecules are going to move to the left
That row of molecules moving to the left.
And the row above it won't fully move to the left just yet
but it will start to get pulled.
Let me just draw all of the 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 will essentially be pulled in the direction
the top row will be pulled in the direction of the bottom row
They'll start moving in that direction
and then the bottom row will recoil back
and then you fast forward a little bit
Then the top row will have moved to the left
Now the bottom row will start to move back
And then 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 solids down here
so it'll move back and you can see this
tranverse wave, this S Wave, propagating.
Essentially right over here the 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 weaker bonds formed
because water is a polar molecule
so the halfway polar sides or halfway positive sides
are somewhat attracted to the halfway negative sides
so they 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.
This one's going to bump into that one
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 can view it as 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
And this guy might have had 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
where this one bumps into that one and goes back
and this one bumps into that one and goes back
and 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 these bonds are also not
strong enough for the adjacent molecules
to help these blue guys to retract
to their original position.
So when I talked about elasticity in the last video
that's what I was talking about.
The bonds aren't strong enough to cause
things that have deformed to move back to
where they are.
And also the bonds aren't strong enough to
allow things that are deformed to pull other things
with it.
And that's why in general S Waves only
travel in solid and they won't travel
in liquid or air.

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