WEBVTT 00:00:00.290 --> 00:00:03.190 The diagram shows the flow of a fluid past a point. 00:00:03.460 --> 00:00:05.900 The lines represent the direction of the fluid flow. 00:00:06.360 --> 00:00:09.510 The black region represents a solid obstacle to the flow. 00:00:10.030 --> 00:00:14.110 Which of the two regions within the dashed lines has a steadier fluid flow? 00:00:14.910 --> 00:00:16.970 Let’s take a closer look at the diagram. 00:00:17.330 --> 00:00:20.160 We have a fluid flowing generally from left to right. 00:00:20.570 --> 00:00:24.100 But we can also see that there’s a small round obstacle in its way. 00:00:24.630 --> 00:00:28.570 Remember that these lines illustrate the motion of different layers of the fluid. 00:00:28.970 --> 00:00:34.080 And as the fluid encounters the obstacle, we see that the fluid has to get redirected around it. 00:00:34.590 --> 00:00:42.030 Thanks to the shape of the obstacle and the properties of whatever type of fluid this is, there aren’t any areas of great turbulence. 00:00:42.440 --> 00:00:48.280 The overall flow is pretty steady, as indicated by the relatively straight and parallel flow lines. 00:00:48.720 --> 00:00:54.540 Recall that “steady” describes a fluid that doesn’t experience much change in speed and direction. 00:00:54.930 --> 00:00:57.160 It’s the opposite of a turbulent flow. 00:00:57.710 --> 00:01:03.810 Of course, we have to keep in mind that we aren’t using any numbers or mathematics to describe the fluid flow here. 00:01:04.330 --> 00:01:10.490 Properties of fluid motion, such as steadiness, are typically very complicated to express quantitatively. 00:01:10.910 --> 00:01:15.590 So here we have to rely on our judgment to describe the flow qualitatively. 00:01:16.160 --> 00:01:23.220 Still, it’s up to us to determine which of these boxed-in regions, I or II, shows steadier fluid flow. 00:01:23.780 --> 00:01:29.880 Sometimes this is very easy to tell, for instance, if one region has really curved flow lines. 00:01:30.290 --> 00:01:35.300 But it might not be as immediately obvious looking at this particular diagram. 00:01:36.140 --> 00:01:44.280 Both regions I and II do show quite steady fluid flow, but one of the regions is a little more steady than the other. 00:01:44.730 --> 00:01:55.060 Now, if somehow we were only shown the insides of these two regions and we couldn’t see the rest of the diagram, we might be more inclined to believe that they have equally steady fluid flow. 00:01:55.580 --> 00:01:58.390 But we have to think about the whole picture, literally. 00:01:59.140 --> 00:02:05.030 Notice that region I is located directly behind the obstacle, which we know is solid. 00:02:05.470 --> 00:02:08.210 And therefore, the fluid must get redirected around it. 00:02:08.440 --> 00:02:11.630 That’s why we see curved flow lines in the surrounding area. 00:02:12.250 --> 00:02:15.840 Now, these curves don’t only indicate a change in direction. 00:02:16.400 --> 00:02:24.100 Recall that the different layers of the fluid, which are represented by these flow lines, exert force on each other, like a sort of friction. 00:02:24.560 --> 00:02:29.310 So, as one layer changes direction, it drags the surrounding layers along with it. 00:02:29.360 --> 00:02:32.660 And we can imagine how this disturbs the general flow of things. 00:02:33.060 --> 00:02:38.260 Fluid particles are bumping into each other, changing direction, and thus slowing down. 00:02:38.720 --> 00:02:47.710 So the fluid around the obstacle is flowing slower, which is actually represented by these lines here being less intense than the bulk of the flow shown in the diagram. 00:02:48.180 --> 00:02:54.470 Knowing this, we can see how region I isn’t quite perfectly steady due to the presence of this obstacle. 00:02:54.970 --> 00:02:58.760 This is a good hint that region II is in fact steadier. 00:02:59.460 --> 00:03:04.550 And then after the obstacle, as the fluid moves along, it eventually evens itself out. 00:03:04.650 --> 00:03:07.430 And there are no more obstructions to mess up the flow. 00:03:07.900 --> 00:03:13.830 In this context, we can further understand how region II has a steadier flow than region I.