WEBVTT 00:00:00.000 --> 00:00:01.839 - [Instructor] Beach balls float on water, 00:00:01.839 --> 00:00:04.510 icebergs float on water, certain things float on water, 00:00:04.510 --> 00:00:07.337 whereas others, like a steel ball, sinks. 00:00:07.337 --> 00:00:10.410 Why? Why do certain things float and certain things sink? 00:00:10.410 --> 00:00:12.114 And also, if you consider the things that are floating, 00:00:12.114 --> 00:00:14.743 sometimes, most of the object is above the water, 00:00:14.743 --> 00:00:17.350 like the beach ball, but when it comes to the iceberg, 00:00:17.350 --> 00:00:20.303 look, most of the object is submerged below the water. 00:00:20.303 --> 00:00:23.085 So what controls how much of that object is above 00:00:23.085 --> 00:00:24.863 and how much of it is submerged? 00:00:24.863 --> 00:00:26.194 Now, you probably know the answer 00:00:26.194 --> 00:00:29.554 has something to do with density, but why density? 00:00:29.554 --> 00:00:31.140 Why does density matter? 00:00:31.140 --> 00:00:33.014 What is the deeper reason behind this? 00:00:33.014 --> 00:00:35.040 That's what we wanna figure out in this video, 00:00:35.040 --> 00:00:35.873 so let's begin. 00:00:35.873 --> 00:00:38.237 So, let's start by submerging an object inside a fluid. 00:00:38.237 --> 00:00:40.234 Let's consider water to be our fluid, 00:00:40.234 --> 00:00:42.155 and let's submerge a cubicle object. 00:00:42.155 --> 00:00:45.606 A cube because it's a nice shape. Easier to analyze, okay? 00:00:45.606 --> 00:00:47.992 We know that the water is going to start pushing 00:00:47.992 --> 00:00:50.602 on this cube in all the directions perpendicular 00:00:50.602 --> 00:00:51.617 to the surface. 00:00:51.617 --> 00:00:53.492 And if we calculate that force per area, 00:00:53.492 --> 00:00:55.108 we call that as the pressure. 00:00:55.108 --> 00:00:56.058 Now, the important thing 00:00:56.058 --> 00:00:58.734 is that the pressure increases with depth. 00:00:58.734 --> 00:01:01.344 So, let's see how that affects the cube. 00:01:01.344 --> 00:01:02.918 First, let's consider the horizontal forces. 00:01:02.918 --> 00:01:04.590 For example, consider the forces on the left 00:01:04.590 --> 00:01:06.117 and the right side of the cube. 00:01:06.117 --> 00:01:07.752 The forces would look somewhat like this. 00:01:07.752 --> 00:01:10.680 Look, the pressure is increasing with the depth, 00:01:10.680 --> 00:01:14.244 but notice the forces pretty much cancel out. 00:01:14.244 --> 00:01:15.093 And the same thing is gonna happen 00:01:15.093 --> 00:01:17.323 with the front and back surface of the cube as well, 00:01:17.323 --> 00:01:19.744 so all the horizontal forces cancel out. 00:01:19.744 --> 00:01:22.243 But what about the top and the bottom surface? 00:01:22.243 --> 00:01:23.240 Well, let's see. 00:01:23.240 --> 00:01:25.106 The pressure on the top is smaller 00:01:25.106 --> 00:01:27.750 than the pressure from the bottom. 00:01:27.750 --> 00:01:31.088 And because the surface area is exactly the same 00:01:31.088 --> 00:01:32.991 on both the top and the bottom, 00:01:32.991 --> 00:01:36.183 the forces from the bottom will be larger 00:01:36.183 --> 00:01:38.679 than the force from the top. 00:01:38.679 --> 00:01:41.085 And therefore, now the forces don't cancel out. 00:01:41.085 --> 00:01:45.570 Instead, there is a net force acting upwards. 00:01:45.570 --> 00:01:48.240 And that net force that's acting upwards 00:01:48.240 --> 00:01:50.989 is what we call the buoyant force. 00:01:50.989 --> 00:01:55.351 This is the force that tends to make things float. 00:01:55.351 --> 00:01:57.927 It tends to make things buoyant. 00:01:57.927 --> 00:01:59.982 That's what the b stands for over here. 00:01:59.982 --> 00:02:02.760 And so, notice where does the buoyant force come from? 00:02:02.760 --> 00:02:06.753 It ultimately comes from the fact that as you go deeper, 00:02:06.753 --> 00:02:09.042 the pressure increases. 00:02:09.042 --> 00:02:11.607 That is where it all comes from. 00:02:11.607 --> 00:02:13.836 Now, although we did this analysis for a cube, 00:02:13.836 --> 00:02:17.495 this would be true for any object of any arbitrary shape. 00:02:17.495 --> 00:02:18.993 Even if you take some kind of a rock, 00:02:18.993 --> 00:02:20.737 which has some random shape, 00:02:20.737 --> 00:02:22.775 the fact is the pressure from the top 00:02:22.775 --> 00:02:25.290 is smaller than the pressure from the bottom. 00:02:25.290 --> 00:02:26.916 And so, when you look at all the forces, 00:02:26.916 --> 00:02:30.495 eventually, there will be a net force acting upward, 00:02:30.495 --> 00:02:32.351 the buoyant force. 00:02:32.351 --> 00:02:33.743 Now comes the big question, 00:02:33.743 --> 00:02:35.996 because of this buoyant force, how do we know 00:02:35.996 --> 00:02:38.601 whether this object is going to float or sink? 00:02:38.601 --> 00:02:42.396 Well, it depends upon the weight of this object. 00:02:42.396 --> 00:02:44.740 If the weight of this object 00:02:44.740 --> 00:02:47.897 is smaller than the buoyant force, 00:02:47.897 --> 00:02:49.737 well, then the buoyant force wins. 00:02:49.737 --> 00:02:51.780 The net force will now be upwards, 00:02:51.780 --> 00:02:54.611 and as a result, the object will accelerate upwards, 00:02:54.611 --> 00:02:56.829 making it float. 00:02:56.829 --> 00:03:01.101 On the other hand, if the weight of the object is larger 00:03:01.101 --> 00:03:04.241 than the buoyant force, then the weight wins 00:03:04.241 --> 00:03:06.657 and therefore, the object will accelerate down, 00:03:06.657 --> 00:03:09.418 in other words, the object will sink, 00:03:09.418 --> 00:03:10.667 which means all we need to do 00:03:10.667 --> 00:03:12.746 is figure out what the buoyant force is. 00:03:12.746 --> 00:03:14.321 If we can do that, we can predict 00:03:14.321 --> 00:03:16.759 whether an object is going to float or sink. 00:03:16.759 --> 00:03:18.792 But how do we figure out what the buoyant force 00:03:18.792 --> 00:03:21.499 is going to be, especially when the objects 00:03:21.499 --> 00:03:23.337 have irregular shapes like this? 00:03:23.337 --> 00:03:25.290 Well, here's a way to think about it. 00:03:25.290 --> 00:03:28.234 Imagine that instead of having a rock, 00:03:28.234 --> 00:03:30.491 if I had a styrofoam over here, 00:03:30.491 --> 00:03:32.743 but of exactly the same shape 00:03:32.743 --> 00:03:36.673 and volume submerged in the same liquid, in the same fluid, 00:03:36.673 --> 00:03:38.809 the question to think about is, 00:03:38.809 --> 00:03:41.709 would the buoyant force now be the same as before 00:03:41.709 --> 00:03:43.289 or would it be different? 00:03:43.289 --> 00:03:45.853 Why don't you pause the video and think about this? 00:03:45.853 --> 00:03:47.497 All right, at first, it might feel like, 00:03:47.497 --> 00:03:49.495 "Hey, I have a different substance altogether, 00:03:49.495 --> 00:03:51.994 so the buoyant force must be different, obviously." 00:03:51.994 --> 00:03:53.018 But think about it, 00:03:53.018 --> 00:03:54.385 the buoyant force, where does it come from? 00:03:54.385 --> 00:03:57.090 It comes from the pressure differences, isn't it? 00:03:57.090 --> 00:03:58.845 And what does pressure depend on? 00:03:58.845 --> 00:04:02.880 Well, the pressure only depends upon the depth. 00:04:02.880 --> 00:04:06.034 And so if you have the exact same shape as before, 00:04:06.034 --> 00:04:09.278 then all the forces will be exactly the same, and therefore, 00:04:09.278 --> 00:04:11.942 the pressure would be exactly the same as before, 00:04:11.942 --> 00:04:13.905 and therefore, the buoyant force 00:04:13.905 --> 00:04:16.141 would be exactly the same as before. 00:04:16.141 --> 00:04:17.677 So, this is the key insight. 00:04:17.677 --> 00:04:19.182 This means that the buoyant force 00:04:19.182 --> 00:04:22.065 has nothing to do with what material you have submerged. 00:04:22.065 --> 00:04:25.879 All that matters is the shape of the material. 00:04:25.879 --> 00:04:27.247 If the shape remains the same, 00:04:27.247 --> 00:04:29.107 then regardless of what material it is, 00:04:29.107 --> 00:04:31.491 the buoyant force should be the same. 00:04:31.491 --> 00:04:34.598 Okay, the buoyant force does not depend upon the material, 00:04:34.598 --> 00:04:36.492 how can we use that insight? 00:04:36.492 --> 00:04:40.141 Well, now, in this exact same shape, 00:04:40.141 --> 00:04:44.562 let's fill water. (laughs) 00:04:44.562 --> 00:04:46.734 Well, the buoyant force is gonna be the same as before, 00:04:46.734 --> 00:04:50.792 but this time, we know that this particular piece 00:04:50.792 --> 00:04:54.159 of water is in equilibrium because, remember, 00:04:54.159 --> 00:04:55.345 this is just water. 00:04:55.345 --> 00:04:57.495 It's static. It's not moving at all. 00:04:57.495 --> 00:05:00.134 This piece of water is neither sinking nor it's floating. 00:05:00.134 --> 00:05:02.938 It's not accelerating upwards, which means it's static. 00:05:02.938 --> 00:05:05.604 And therefore, this means this piece of water 00:05:05.604 --> 00:05:07.328 is in equilibrium. 00:05:07.328 --> 00:05:10.290 So, the weight of this piece of water 00:05:10.290 --> 00:05:13.286 must be exactly equal 00:05:13.286 --> 00:05:15.238 to the buoyant force. 00:05:15.238 --> 00:05:16.360 And there we have it. 00:05:16.360 --> 00:05:20.347 We have figured out what the buoyant force must be. 00:05:20.347 --> 00:05:21.846 For this particular shape, 00:05:21.846 --> 00:05:23.859 it doesn't matter what you put inside this, 00:05:23.859 --> 00:05:26.875 the buoyant force should equal the weight 00:05:26.875 --> 00:05:29.737 of this fluid. 00:05:29.737 --> 00:05:32.488 In other words, when you fill this object, 00:05:32.488 --> 00:05:35.760 when you fill this space with some object, 00:05:35.760 --> 00:05:39.270 that fluid got displaced somewhere, 00:05:39.270 --> 00:05:43.996 and the weight of that fluid that got displaced literally 00:05:43.996 --> 00:05:46.598 is the buoyant force. 00:05:46.598 --> 00:05:47.916 Isn't it amazing? 00:05:47.916 --> 00:05:50.853 Long story short, the buoyant force acting on any object 00:05:50.853 --> 00:05:55.410 will always equal the weight of the fluid that it displaces. 00:05:55.410 --> 00:05:59.557 And this is what we call the Archimedes' principle. 00:05:59.557 --> 00:06:01.587 So now let's see if we can use this insight 00:06:01.587 --> 00:06:03.556 to figure out when will an object float 00:06:03.556 --> 00:06:05.600 and when it'll sink, okay? 00:06:05.600 --> 00:06:07.566 So, we know that if the weight of the object 00:06:07.566 --> 00:06:10.332 is larger than the buoyant force, in other words, 00:06:10.332 --> 00:06:12.993 larger than the weight of the fluid it displaces, 00:06:12.993 --> 00:06:14.743 the object is going to sink. 00:06:14.743 --> 00:06:17.495 So this is a sinking case, but when will this happen? 00:06:17.495 --> 00:06:20.238 When will the object have more weight compared to the fluid 00:06:20.238 --> 00:06:21.486 that it's displacing? 00:06:21.486 --> 00:06:22.990 Well, what is weight? 00:06:22.990 --> 00:06:24.685 Weight is just mass times gravity. 00:06:24.685 --> 00:06:26.838 So we can plug mg over here. 00:06:26.838 --> 00:06:28.325 This represents the mass of the object, 00:06:28.325 --> 00:06:30.643 and this represents the mass of the fluid 00:06:30.643 --> 00:06:31.993 that got displaced. 00:06:31.993 --> 00:06:35.555 But we know that mass is the density times volume. 00:06:35.555 --> 00:06:36.638 So we can replace masses 00:06:36.638 --> 00:06:39.799 with density of the object times the volume of the object, 00:06:39.799 --> 00:06:41.811 and the density of the fluid displaced 00:06:41.811 --> 00:06:43.597 times the volume of the fluid displaced. 00:06:43.597 --> 00:06:45.232 But here's the key thing, 00:06:45.232 --> 00:06:48.368 the volume of the fluid displaced is exactly the same 00:06:48.368 --> 00:06:51.060 as the volume of the object, right? 00:06:51.060 --> 00:06:53.239 And therefore, these things cancel out, 00:06:53.239 --> 00:06:55.875 and look what the condition becomes. 00:06:55.875 --> 00:06:56.893 The condition for sinking 00:06:56.893 --> 00:06:59.576 is the density of the object should be larger 00:06:59.576 --> 00:07:01.395 than the density of the fluid it's submerged in. 00:07:01.395 --> 00:07:04.810 When that happens, the object will sink. 00:07:04.810 --> 00:07:06.997 But can we now understand why? 00:07:06.997 --> 00:07:08.661 Well, because if the density of the object 00:07:08.661 --> 00:07:10.583 is larger than that of the fluid, 00:07:10.583 --> 00:07:12.576 then the weight of the object 00:07:12.576 --> 00:07:14.894 will be larger than the weight of the fluid 00:07:14.894 --> 00:07:17.730 that it displaces when it's completely submerged. 00:07:17.730 --> 00:07:19.923 And as a result, its weight wins. 00:07:19.923 --> 00:07:21.870 Its weight will be larger than the buoyant force 00:07:21.870 --> 00:07:24.570 and that's why it ends up sinking. 00:07:24.570 --> 00:07:26.545 And this is why a steel ball sinks in water 00:07:26.545 --> 00:07:29.501 because it has a higher density than water. 00:07:29.501 --> 00:07:31.383 But what if an object has a lower density 00:07:31.383 --> 00:07:33.007 than the fluid it's submerged in? 00:07:33.007 --> 00:07:35.644 Well, then its weight would be lower 00:07:35.644 --> 00:07:38.100 than the weight of the fluid that it displaces, 00:07:38.100 --> 00:07:39.740 and therefore, the buoyant force will be larger 00:07:39.740 --> 00:07:41.066 and the whole object will accelerate up. 00:07:41.066 --> 00:07:44.164 In other words, this is the condition for flotation. 00:07:44.164 --> 00:07:45.806 This is the reason why beach balls 00:07:45.806 --> 00:07:47.509 and icebergs float in water 00:07:47.509 --> 00:07:50.745 because they have a density that's smaller than water. 00:07:50.745 --> 00:07:52.195 And now, look, we have the complete equation 00:07:52.195 --> 00:07:53.353 for the buoyant force. 00:07:53.353 --> 00:07:55.578 The buoyant force is the weight of the fluid displaced, 00:07:55.578 --> 00:07:59.890 which is the density of the fluid times the volume times g. 00:07:59.890 --> 00:08:02.230 And so this is the expression for the buoyant force. 00:08:02.230 --> 00:08:03.352 And again, this helps us see 00:08:03.352 --> 00:08:04.995 why it's the density that matters 00:08:04.995 --> 00:08:06.484 because, look, the weight of the displaced fluid 00:08:06.484 --> 00:08:08.488 will have the exact same volume 00:08:08.488 --> 00:08:10.605 as the weight of the submerged object. 00:08:10.605 --> 00:08:11.742 The volumes cancel out, 00:08:11.742 --> 00:08:14.245 and that's why it's eventually the density that decides 00:08:14.245 --> 00:08:16.100 whether something's going to sink or float. 00:08:16.100 --> 00:08:18.402 So it all comes from the Archimedes' principle, 00:08:18.402 --> 00:08:21.780 which eventually comes from the pressure differences. 00:08:21.780 --> 00:08:22.647 Amazing, right? 00:08:22.647 --> 00:08:24.240 But now, let's think about what's going to happen 00:08:24.240 --> 00:08:25.248 to this object. 00:08:25.248 --> 00:08:27.671 We know it's going to start accelerating up, 00:08:27.671 --> 00:08:29.490 but when will it stop? 00:08:29.490 --> 00:08:30.356 Well, let's see. 00:08:30.356 --> 00:08:32.740 As long as the whole thing is submerged inside, 00:08:32.740 --> 00:08:34.809 the buoyant force stays the same. 00:08:34.809 --> 00:08:38.943 But now, as it starts coming above the surface, 00:08:38.943 --> 00:08:42.465 it starts displacing lesser fluid. 00:08:42.465 --> 00:08:43.413 Can you see that? 00:08:43.413 --> 00:08:46.170 It now only displaces this much amount of the fluid, 00:08:46.170 --> 00:08:49.489 and therefore, the buoyant force will become smaller. 00:08:49.489 --> 00:08:51.612 But as long as it's still larger than the weight, 00:08:51.612 --> 00:08:55.200 the object continues accelerating upwards, 00:08:55.200 --> 00:08:57.483 and eventually, at some point, 00:08:57.483 --> 00:08:59.037 the weight of the fluid displaced 00:08:59.037 --> 00:09:02.430 will exactly match the weight of the object, 00:09:02.430 --> 00:09:04.295 and that's when equilibrium is reached 00:09:04.295 --> 00:09:06.595 and the object pretty much stops at that point. 00:09:06.595 --> 00:09:08.497 So look, for an object to float, 00:09:08.497 --> 00:09:11.090 it needs to be submerged enough so that the weight 00:09:11.090 --> 00:09:15.216 of the fluid displaced exactly equals the object's weight. 00:09:15.216 --> 00:09:16.252 Does that make sense? 00:09:16.252 --> 00:09:17.350 Now, consider the beach ball. 00:09:17.350 --> 00:09:21.753 It has a very low density, so the amount of water needed 00:09:21.753 --> 00:09:25.252 to match the weight of the beach ball is very little. 00:09:25.252 --> 00:09:27.504 So only a small portion needs to be submerged 00:09:27.504 --> 00:09:29.238 because by then, the weight of the water 00:09:29.238 --> 00:09:32.491 that has been displaced already equals the weight 00:09:32.491 --> 00:09:33.989 of the entire beach ball 00:09:33.989 --> 00:09:37.669 because the beach ball has a much smaller density compared 00:09:37.669 --> 00:09:38.544 to the water. 00:09:38.544 --> 00:09:39.835 That's the whole idea. 00:09:39.835 --> 00:09:42.241 On the other hand, if you consider an iceberg, 00:09:42.241 --> 00:09:46.647 it has a density very, very close to that of water. 00:09:46.647 --> 00:09:49.738 And so now, to displace the water equal to its weight, 00:09:49.738 --> 00:09:52.834 you need to submerge a lot more. 00:09:52.834 --> 00:09:55.495 Pretty much the entire thing needs to be submerged 00:09:55.495 --> 00:09:58.662 because its density is very, very close to that of water. 00:09:58.662 --> 00:10:00.351 It's still less, that's why it's floating, 00:10:00.351 --> 00:10:01.936 but it's very close to that of water. 00:10:01.936 --> 00:10:04.483 So, higher the density of the object, 00:10:04.483 --> 00:10:06.836 more it needs to be submerged 00:10:06.836 --> 00:10:09.238 so that it can be in equilibrium. 00:10:09.238 --> 00:10:10.743 So, if you put it all together, 00:10:10.743 --> 00:10:12.410 we can say that when the density of an object 00:10:12.410 --> 00:10:15.588 is smaller than density of the fluid, it will be floating. 00:10:15.588 --> 00:10:17.641 And the amount of the objects submerged depends 00:10:17.641 --> 00:10:19.498 on how small the density is. 00:10:19.498 --> 00:10:21.113 If the density of the object is very tiny compared 00:10:21.113 --> 00:10:22.530 to the density of the fluid, 00:10:22.530 --> 00:10:23.739 it'll be submerged just a little bit. 00:10:23.739 --> 00:10:25.659 On the other hand, if the density is very close 00:10:25.659 --> 00:10:27.223 to the density of the fluid, but still smaller, 00:10:27.223 --> 00:10:29.040 it needs to be smaller for it to be floating, 00:10:29.040 --> 00:10:31.742 but if it's close enough, then most of it will be submerged. 00:10:31.742 --> 00:10:34.503 On the other hand, if the density of the object is higher 00:10:34.503 --> 00:10:35.853 than the density of the fluid itself, 00:10:35.853 --> 00:10:37.244 then the object will sink. 00:10:37.244 --> 00:10:38.743 And by the way, in all the examples, 00:10:38.743 --> 00:10:40.242 we have taken water as our fluid, 00:10:40.242 --> 00:10:42.653 but this would work for any fluid, okay? 00:10:42.653 --> 00:10:44.153 For example, air is also a fluid, 00:10:44.153 --> 00:10:46.120 so it can also put a buoyant force. 00:10:46.120 --> 00:10:48.253 But since air has a very low density compared 00:10:48.253 --> 00:10:50.825 to everyday objects, we don't usually notice it. 00:10:50.825 --> 00:10:53.738 But if you consider a helium balloon, its density, 00:10:53.738 --> 00:10:56.142 helium density is smaller than that of the air. 00:10:56.142 --> 00:10:58.993 And now, as a result of that, helium tends to float, 00:10:58.993 --> 00:11:01.499 and that's why helium balloons tend to rise up. 00:11:01.499 --> 00:11:03.107 But now is an interesting question, 00:11:03.107 --> 00:11:06.428 what if the density of the object exactly equals the density 00:11:06.428 --> 00:11:07.304 of the fluid? 00:11:07.304 --> 00:11:08.250 What happens then? 00:11:08.250 --> 00:11:10.243 Well, now, if you completely submerge it, 00:11:10.243 --> 00:11:13.598 it will neither float nor sink. 00:11:13.598 --> 00:11:16.343 We call this the neutral buoyancy. 00:11:16.343 --> 00:11:20.489 That object will just stay at that particular depth, 00:11:20.489 --> 00:11:22.620 and that's exactly how submarines 00:11:22.620 --> 00:11:24.744 can stay at a particular depth. 00:11:24.744 --> 00:11:28.340 They do that by changing their average density. 00:11:28.340 --> 00:11:29.848 If they want to sink, 00:11:29.848 --> 00:11:31.845 they will increase their average density 00:11:31.845 --> 00:11:36.231 by allowing water to flood their tanks. 00:11:36.231 --> 00:11:37.064 On the other hand, 00:11:37.064 --> 00:11:40.489 if it now needs to come towards the surface of the water, 00:11:40.489 --> 00:11:42.491 then it'll decrease its average density. 00:11:42.491 --> 00:11:44.996 It does so by now using compressed air 00:11:44.996 --> 00:11:47.231 to force the water out. 00:11:47.231 --> 00:11:49.614 And by carefully balancing the amount of water 00:11:49.614 --> 00:11:51.422 and air inside its tanks, 00:11:51.422 --> 00:11:54.740 it can also make sure that its density exactly equals out 00:11:54.740 --> 00:11:57.740 of the surrounding water, maintaining neutral buoyancy, 00:11:57.740 --> 00:12:00.493 and in which case, it'll stay at a particular depth. 00:12:00.493 --> 00:12:02.115 That's amazing, right? 00:12:02.115 --> 00:12:04.901 Now, instead of submarine, imagine you were wearing a suit 00:12:04.901 --> 00:12:07.843 which could also maintain neutral buoyancy in water. 00:12:07.843 --> 00:12:10.182 Then you would be, just like the submarine, 00:12:10.182 --> 00:12:12.482 stay at a particular location in water, 00:12:12.482 --> 00:12:14.895 not going down nor going up. 00:12:14.895 --> 00:12:18.495 In other words, you would kind of feel weightless, 00:12:18.495 --> 00:12:20.106 which means now you can train 00:12:20.106 --> 00:12:22.490 for the weightless environments that you would face 00:12:22.490 --> 00:12:23.828 in the space stations. 00:12:23.828 --> 00:12:24.999 And that's exactly 00:12:24.999 --> 00:12:28.614 what the NASA's Neutral Buoyancy Lab does. 00:12:28.614 --> 00:12:30.593 It's a giant swimming pool inside 00:12:30.593 --> 00:12:33.600 which astronauts can maintain neutral buoyancy 00:12:33.600 --> 00:12:34.872 and train for the weightless environment. 00:12:34.872 --> 00:12:37.182 It's not perfect, but it's way better 00:12:37.182 --> 00:12:40.323 and more realistic than training on the ground, for example.