Seismic waves and how we know Earth's structure
Why S-Waves Only Travel in Solids Why S-Waves Only Travel in Solids
⇐ Use this menu to view and help create subtitles for this video in many different languages.
You'll probably want to hide YouTube's captions if using these subtitles.
- 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
Be specific, and indicate a time in the video:
At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
|
Have something that's not a question about this content? |
This discussion area is not meant for answering homework questions.
Discuss the site
For general discussions about Khan Academy, visit our Reddit discussion page.
Flag inappropriate posts
Here are posts to avoid making. If you do encounter them, flag them for attention from our Guardians.
abuse
- disrespectful or offensive
- an advertisement
not helpful
- low quality
- not about the video topic
- soliciting votes or seeking badges
- a homework question
- a duplicate answer
- repeatedly making the same post
wrong category
- a tip or feedback in Questions
- a question in Tips & Feedback
- an answer that should be its own question
about the site
Share a tip
Suggest a fix
Have something that's not a tip or feedback about this content?
This discussion area is not meant for answering homework questions.