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.