Cosmology and astronomy
Why S-waves only travel in solids
Why S-Waves Only Travel in Solids. Created by Sal Khan.
Want to join the conversation?
- Would s-waves not travel through liquids and gases because they are not as malleable as most solids?(10 votes)
- yes, and also it is easier for the P waves to bounce off soild than liquids.(4 votes)
- Can someone please give me a quick, simple explanation on why s-waves only travel through solids? This video is sort of confusing :/
Thank you!(6 votes)
- Sal is explaining why s-waves don't travel through water or air on the molecular level...It might help to watch the previous video again. However, try thinking of it this way. When you have an earthquake or big explosion of some sort it is going to cause both primary and secondary body waves right. At this point you know that the p-waves will travel through both solids and liquids and the s-waves won't. This is where the previous video comes in handy. You can visually (in the video) see that p-waves look like the are traveling just parallel to where the explosion or earthquake took place. S-waves look like they are traveling perpendicular to the explosion or earthquake. Now, the bond strength in solids versus liquids and gasses explain why s-waves can only travel through solids. (think Newtonian physics in regards to a solid...every action has an equal and opposite reaction). Liquids and gasses can't transmit these waves in the same way because the bonds don't hold together and carry the s-waves in this perpendicular looking fashion. They just break and slide past each other transmitting a p-wave. I hope this helps. Again watch the previous video for visual differences of what the hammer is doing when it strikes the rock.(6 votes)
- Can S-waves travel through non-newtonian fluids?(4 votes)
- So if both S-waves, and P-waves are being produced, when the S-waves hit something liquid, what happens to them? If they can't travel though liquid then are they converted into P-waves? And if that is true, then do areas of the earth that don't receive S-waves during a particular earthquake or explosion receive even MORE P-waves then the rest of the Earth?(3 votes)
- If Sal "hit" the earth on the side, wouldn't it just make a compression/p wave in the direction of the hit? Or is the origin of an s wave more complex?(2 votes)
- Yes, if somebody "hit" the Earth on one side it will transmit p-waves in the direction of the hit, but it will also transmit s-waves. What he is explaining here is why we the s-waves are not going to travel through liquids or gasses. Therefore, when we detect the p and s-waves on the other side of the Earth we can calculate what the mechanical composition of our planet is.(1 vote)
- What are the covalent bonds?(1 vote)
- Valence electrons are the electrons on the outermost shell, covalent means that these electrons are shared.(2 votes)
- I didn't quite get it... S-waves only travel through solids because the bonds of the molecules in liquids are weaker?(2 votes)
- yes. The bonds break before you can get the wave going, so the actual wave would never occur.(1 vote)
- Sorry I don't get how these are transverse s-waves when you're transferring energy from right to left and expecting the particles to oscillate parallel to the direction of energy transfer. You seem to have explained that the energy from longitudinal p-waves is absorbed in solids faster than in fluids. (as the energy is used to yank other particles out of alignment)(2 votes)
- I think your on the right track but have reversed what's going on...transverse s-waves are not oscillating particles parallel to the direction of energy. That is your p-wave.(1 vote)
- Maybe this is a silly question, but I just wanted to be sure.
In the video, Sal shows the row of molecules (in the solid) above the ones he hit with the hammer get moved to left.
There isn't any difference between what he's drawing as up and down, right? So the row of molecules below would also move, and the wave would propagate in both directions?(2 votes)
- Sound waves travel in solids as transverse or longtudinal(1 vote)
- Solids can transmit transverse and longitudinal waves.
Whether you want to call that "sound" is the question.
Sound is what you hear in your ears. It is the result of longitudinal waves through the air, hitting your ear.
You can send transverse waves through a solid but you can't stick your ear in a solid to "hear" them.
I don't think I'd call that sound.(2 votes)
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.