1 00:00:15,966 --> 00:00:16,900 And why this matters. 2 00:00:16,900 --> 00:00:19,602 Because in fact, this is one of the things, 3 00:00:19,602 --> 00:00:22,939 all the way back to Hook and his cannonballs, 4 00:00:22,939 --> 00:00:26,776 and even way before that, people noticed this stuff. 5 00:00:26,776 --> 00:00:29,346 I broke something, but it always seems 6 00:00:29,346 --> 00:00:31,915 to slice in a certain way. 7 00:00:31,915 --> 00:00:32,515 Why is that? 8 00:00:32,515 --> 00:00:36,553 Why would something always break in the same way every time? 9 00:00:36,553 --> 00:00:38,388 With the same kind of-- 10 00:00:38,388 --> 00:00:40,056 what does way mean here? 11 00:00:40,056 --> 00:00:42,125 It means like the angles that form. 12 00:00:42,125 --> 00:00:45,996 So why are those angles always kind of the same? 13 00:00:45,996 --> 00:00:48,198 Well, it has to do with the strength, 14 00:00:48,198 --> 00:00:51,034 the relative strength, of the planes 15 00:00:51,034 --> 00:00:53,570 inside of these crystals. 16 00:00:53,570 --> 00:00:55,939 And in fact, here's a beautiful piece 17 00:00:55,939 --> 00:00:58,108 of work on stretching a wire. 18 00:00:58,108 --> 00:01:02,312 All right, so here you're taking a wire that is crystalline 19 00:01:02,312 --> 00:01:03,813 and you're pulling it. 20 00:01:03,813 --> 00:01:05,682 And you can see how it breaks. 21 00:01:05,682 --> 00:01:07,350 You can actually see how it breaks. 22 00:01:07,350 --> 00:01:10,387 It's breaking along the weakest plane. 23 00:01:10,387 --> 00:01:11,988 It's breaking along the weakest plane. 24 00:01:11,988 --> 00:01:14,289 So if you want to think, then, about well, OK, 25 00:01:14,289 --> 00:01:18,027 how do I know how to make this stronger, for example? 26 00:01:18,027 --> 00:01:20,096 So it stops breaking-- you know, the weakest link 27 00:01:20,096 --> 00:01:22,399 is the weakest link. 28 00:01:22,399 --> 00:01:22,999 That was deep. 29 00:01:26,703 --> 00:01:28,670 So then that's the plane of the crystal 30 00:01:28,670 --> 00:01:29,806 that I need to think about. 31 00:01:29,806 --> 00:01:34,811 How are bonding together or packing in in that plane? 32 00:01:34,811 --> 00:01:35,878 This is real stuff. 33 00:01:35,878 --> 00:01:39,582 This comes about in many of the properties, not just 34 00:01:39,582 --> 00:01:44,888 the fracture, but in many of the properties of these materials. 35 00:01:44,888 --> 00:01:49,225 The property, as I said before, depends heavily on which plane 36 00:01:49,225 --> 00:01:50,060 you're in. 37 00:01:50,060 --> 00:01:52,529 And we'll see that more and more.