1 00:00:00,783 --> 00:00:03,389 In the last video I gave a little bit of a 2 00:00:03,389 --> 00:00:06,243 hand wavey explanation about why S-Waves 3 00:00:06,243 --> 00:00:09,365 don't travel in liquid or air. 4 00:00:09,365 --> 00:00:10,409 What I want to do in this video 5 00:00:10,409 --> 00:00:12,459 is give you a little bit more intuitive understanding 6 00:00:12,459 --> 00:00:14,867 of that, it'd really go down to the molecular level. 7 00:00:14,867 --> 00:00:18,361 So let's draw a solid. 8 00:00:18,361 --> 00:00:22,547 And it has nice covalent bonds, strong bonds 9 00:00:22,547 --> 00:00:24,446 between the different molecules. 10 00:00:24,446 --> 00:00:27,212 And the bonds are drawn by these lines in between. 11 00:00:27,212 --> 00:00:29,580 So if I were to hit this solid 12 00:00:29,580 --> 00:00:31,568 with this really small hammer, 13 00:00:31,568 --> 00:00:33,377 just hit it at a molecular level 14 00:00:33,377 --> 00:00:37,944 If I were to hit these molecules hard enough 15 00:00:37,944 --> 00:00:41,612 that they move but not so hard that it breaks the bonds 16 00:00:41,612 --> 00:00:45,111 then what it's going to look like is 17 00:00:45,111 --> 00:00:49,372 this row of molecules are going to move to the left 18 00:00:49,372 --> 00:00:51,873 That row of molecules moving to the left. 19 00:00:51,873 --> 00:00:55,670 And the row above it won't fully move to the left just yet 20 00:00:55,670 --> 00:00:57,894 but it will start to get pulled. 21 00:00:57,894 --> 00:01:01,301 Let me just draw all of the bonds. 22 00:01:01,301 --> 00:01:03,495 Because these are strong bonds that we have 23 00:01:03,495 --> 00:01:07,384 in a solid, actually they could be 24 00:01:07,384 --> 00:01:11,044 ionic bonds as well, because they are strong bonds 25 00:01:11,044 --> 00:01:13,242 that we have in this solid, 26 00:01:13,242 --> 00:01:16,169 they will essentially be pulled in the direction 27 00:01:16,169 --> 00:01:21,442 the top row will be pulled in the direction of the bottom row 28 00:01:21,442 --> 00:01:23,752 They'll start moving in that direction 29 00:01:23,752 --> 00:01:26,476 and then the bottom row will recoil back 30 00:01:26,476 --> 00:01:28,472 and then you fast forward a little bit 31 00:01:28,472 --> 00:01:33,416 Then the top row will have moved to the left 32 00:01:33,416 --> 00:01:38,190 Now the bottom row will start to move back 33 00:01:38,190 --> 00:01:43,009 And then the bottom row will start to move back 34 00:01:43,009 --> 00:01:45,447 especially because, remember, it's bonded to 35 00:01:45,447 --> 00:01:48,072 other things down here. 36 00:01:48,072 --> 00:01:49,685 It's bonded to more of the solids down here 37 00:01:49,685 --> 00:01:51,278 so it'll move back and you can see this 38 00:01:51,278 --> 00:01:55,148 tranverse wave, this S Wave, propagating. 39 00:01:55,148 --> 00:01:58,435 Essentially right over here the peak of the S Wave 40 00:01:58,435 --> 00:02:00,847 is here, now it has moved up. 41 00:02:00,847 --> 00:02:02,859 Now let's think about the exact 42 00:02:02,859 --> 00:02:04,222 same situation with the liquids. 43 00:02:04,222 --> 00:02:05,595 In liquids you don't have these strong 44 00:02:05,595 --> 00:02:07,879 ionic or covalent bonds between the 45 00:02:07,879 --> 00:02:09,410 different molecules. 46 00:02:09,410 --> 00:02:11,276 You just have these weak kind of bonds 47 00:02:11,276 --> 00:02:13,515 usually formed due to polarity 48 00:02:13,515 --> 00:02:17,983 so in a liquid, water's a good example, 49 00:02:17,983 --> 00:02:20,752 you just have these weaker bonds formed 50 00:02:20,752 --> 00:02:24,506 because water is a polar molecule 51 00:02:24,506 --> 00:02:28,311 so the halfway polar sides or halfway positive sides 52 00:02:28,311 --> 00:02:30,934 are somewhat attracted to the halfway negative sides 53 00:02:30,934 --> 00:02:33,414 so they flow past each other 54 00:02:33,414 --> 00:02:36,683 But if I were to hit these water molecules right here 55 00:02:36,683 --> 00:02:39,390 with my hammer, what would happen? 56 00:02:39,390 --> 00:02:41,458 Well they're definitely going to start 57 00:02:41,458 --> 00:02:42,637 moving to the left. 58 00:02:42,637 --> 00:02:44,499 This one's going to bump into that one 59 00:02:44,499 --> 00:02:45,560 which is going to bump into that one... 60 00:02:45,560 --> 00:02:51,380 They're going to move to the left 61 00:02:51,380 --> 00:02:52,564 But these molecules aren't 62 00:02:52,564 --> 00:02:55,279 going to move with them 63 00:02:55,279 --> 00:02:57,107 You can view it as going to break 64 00:02:57,107 --> 00:02:58,441 that very weak bond due to polarity 65 00:02:58,441 --> 00:03:00,931 they're going to move away from each other 66 00:03:00,931 --> 00:03:03,235 Let me draw these top molecules in green. 67 00:03:03,235 --> 00:03:07,577 They're essentially just going to flow past each other 68 00:03:07,577 --> 00:03:09,598 And this guy might have had weak bonds 69 00:03:09,598 --> 00:03:11,435 with stuff below it too. 70 00:03:11,435 --> 00:03:15,343 I should draw it as dotted lines 71 00:03:15,343 --> 00:03:19,694 But because of the impact here 72 00:03:19,694 --> 00:03:20,858 these guys are just going to flow 73 00:03:20,858 --> 00:03:22,933 they're actually going to compress in this direction 74 00:03:22,933 --> 00:03:25,197 You're going to have a P Wave, a compression wave 75 00:03:25,197 --> 00:03:28,243 where this one bumps into that one and goes back 76 00:03:28,243 --> 00:03:29,627 and this one bumps into that one and goes back 77 00:03:29,627 --> 00:03:30,976 and this one bumps into that one 78 00:03:30,976 --> 00:03:34,772 but the bonds aren't strong enough 79 00:03:34,772 --> 00:03:36,930 and it's even more the case with air 80 00:03:36,930 --> 00:03:38,540 But the bonds aren't strong enough for 81 00:03:38,540 --> 00:03:39,975 these blue guys to take these green guys 82 00:03:39,975 --> 00:03:42,213 for a ride. And these bonds are also not 83 00:03:42,213 --> 00:03:46,359 strong enough for the adjacent molecules 84 00:03:46,359 --> 00:03:49,611 to help these blue guys to retract 85 00:03:49,611 --> 00:03:51,477 to their original position. 86 00:03:51,477 --> 00:03:54,114 So when I talked about elasticity in the last video 87 00:03:54,114 --> 00:03:55,199 that's what I was talking about. 88 00:03:55,199 --> 00:03:57,492 The bonds aren't strong enough to cause 89 00:03:57,492 --> 00:03:59,638 things that have deformed to move back to 90 00:03:59,638 --> 00:04:00,662 where they are. 91 00:04:00,662 --> 00:04:02,452 And also the bonds aren't strong enough to 92 00:04:02,452 --> 00:04:04,529 allow things that are deformed to pull other things 93 00:04:04,529 --> 00:04:07,140 with it. 94 00:04:07,140 --> 00:04:09,246 And that's why in general S Waves only 95 00:04:09,246 --> 00:04:11,844 travel in solid and they won't travel 96 00:04:11,844 --> 99:59:59,999 in liquid or air.