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Current time:0:00Total duration:4:15

Video transcript

in the last video I kind of I gave a little bit of a hand wavy explanation about why s ways don't travel in liquid or air and 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 so let's draw a solid and it has nice it has nice covalent bond strong bonds between the different molecules and the bonds are drawn by these lines in between so if I were to hit if I were to hit this solid and you know this really small hammer right I just hit it at a molecular level but if I were to hit these molecules if I were to hit these molecules hard enough so that they move but not so hard enough that it breaks the bonds then and then next then essentially what it's going to look like is these this kind of row of molecules are going to move to the left so you're gonna 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 the same bonds because these are strong bonds that we have in a solid because these are strong bonds actually they could be ionic bonds as well because they are strong bonds that we have in this solid they'll essentially be pulled they'll be pulled in the direction the top role be pulled in the direction of the bottom row and then 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 the top row will have moved to the left and now the bottom row will start to move back and then the but the bottom row will start to kind of move back especially because remember it's bonded it's bonded to other things down here it's bonded to more of the solid down here so to move back and you can see this transverse wave you can see this s wave propagating essentially right over here the S wave 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 you and liquid lets you know water is a good example you just have these kind of weaker bonds formed because of because water is a polar molecule so the kind of halfway polar sides or the halfway positive sides are somewhat attracted to the halfway 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 gonna they're gonna start moving to the left and they're actually you know this one's gonna put a bump into that one which is gonna bump into that which is gonna bump into that one they're gonna move to the left so they're gonna move to the left but these molecules aren't going to move with them it's essentially going to you could view it's gonna break that very weak bond due to polarity they're gonna move away from each other they're essentially let me draw this 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 just dotted lines with stuff below it too but because of because of the the the impact here these guys are just gonna flow they're actually going to compress in this direction you're gonna have a P wave a compression wave go in this direction where this one bumps into that one and then goes back then this one bumps into that one and goes back and this one bumps into that one but there's the bonds aren't strong enough the bonds 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 ride and the bonds are also not strong enough for the guys 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 there's no the bonds aren't strong enough to cause the things that are deformed to kind of move back to where they were and also their bonds aren't strong enough to a lot of the things that are deformed to pull other things with it and so that's why in general S waves don't trap they only travel in solid and they won't travel in liquid or air