WEBVTT 00:00:14.068 --> 00:00:16.368 If humans could fly, 00:00:16.392 --> 00:00:17.819 without tools and machines, 00:00:17.843 --> 00:00:20.102 how fast do you think we would go? 00:00:20.126 --> 00:00:22.509 As of 2012, the world record 00:00:22.533 --> 00:00:24.606 for fastest short-distance sprint speed 00:00:24.630 --> 00:00:27.339 is roughly 27 miles per hour. 00:00:27.363 --> 00:00:29.236 Running speed depends on how much force 00:00:29.260 --> 00:00:31.114 is exerted by the runner's legs, 00:00:31.138 --> 00:00:34.341 and according to Newton's Second Law of Motion, 00:00:34.365 --> 00:00:38.580 force is the product of mass times acceleration. 00:00:38.604 --> 00:00:40.346 And Newton's Third Law states 00:00:40.370 --> 00:00:41.623 that for every action, 00:00:41.647 --> 00:00:44.759 there is an equal and opposite reaction. 00:00:44.783 --> 00:00:46.744 So, that means running requires 00:00:46.768 --> 00:00:48.546 having a ground to push off from, 00:00:48.570 --> 00:00:52.679 and the ground pushes back against the runner's foot. 00:00:52.703 --> 00:00:55.210 So, flying would actually be 00:00:55.234 --> 00:00:57.390 more similar to swimming. 00:00:57.414 --> 00:01:01.317 Michael Phelps is currently the fastest human in water 00:01:01.341 --> 00:01:04.964 and the most decorated Olympian of all time. 00:01:04.988 --> 00:01:06.690 Guess how fast he swims? 00:01:06.714 --> 00:01:09.359 The answer may surprise you. 00:01:09.383 --> 00:01:11.236 His fastest recorded speed is 00:01:11.260 --> 00:01:15.089 less than 5 miles per hour. 00:01:15.113 --> 00:01:17.368 A child on the ground can easily outrun 00:01:17.392 --> 00:01:18.660 Michael Phelps in water, 00:01:18.684 --> 00:01:20.537 but why is that? 00:01:20.561 --> 00:01:23.224 Well, let's go back to Newton's Third Law of Motion. 00:01:23.248 --> 00:01:24.792 When we run, we move forward 00:01:24.816 --> 00:01:27.087 by pushing against the ground with our feet 00:01:27.111 --> 00:01:28.397 and the ground pushes back, 00:01:28.421 --> 00:01:30.388 propelling us forward. 00:01:30.412 --> 00:01:32.591 The ground is solid. 00:01:32.615 --> 00:01:34.846 By definition, it means the particles 00:01:34.870 --> 00:01:37.181 are essentially locked into place 00:01:37.205 --> 00:01:40.431 and must push back instead of getting out of the way, 00:01:40.455 --> 00:01:43.644 but water is liquid and flows easily. 00:01:43.668 --> 00:01:44.730 When we move our limbs 00:01:44.754 --> 00:01:46.231 to push back against the water, 00:01:46.255 --> 00:01:47.636 a part of the water molecules 00:01:47.660 --> 00:01:49.403 can just slide past one another 00:01:49.427 --> 00:01:50.894 instead of pushing back. 00:01:50.918 --> 00:01:52.528 Now, let's think about flying. 00:01:52.552 --> 00:01:54.448 Air has a lot more free space 00:01:54.472 --> 00:01:56.863 for particles to move past one another, 00:01:56.887 --> 00:01:59.910 so even more of our energy would be wasted. 00:01:59.934 --> 00:02:02.176 We would need to push a lot of air backwards 00:02:02.200 --> 00:02:03.977 in order to move forward. 00:02:04.001 --> 00:02:05.833 Astronauts move around in shuttles 00:02:05.857 --> 00:02:07.905 in zero gravity when they're in outer space 00:02:07.929 --> 00:02:10.915 by pulling on handles installed on the ceiling walls 00:02:10.939 --> 00:02:12.644 and floors of the shuttle. 00:02:12.668 --> 00:02:16.470 Now, imagine you were given the ability to float. 00:02:16.494 --> 00:02:19.263 How would you move around in the middle of the street? 00:02:19.287 --> 00:02:20.764 Well, you wouldn't get very far 00:02:20.788 --> 00:02:23.036 by swimming in air, would you? 00:02:23.060 --> 00:02:24.385 Nah, I don't think so! 00:02:24.409 --> 00:02:26.979 Now, assuming you were granted the ability to float 00:02:27.003 --> 00:02:29.733 and the speed to move around efficiently, 00:02:29.757 --> 00:02:32.600 let's discuss the height of your flight. 00:02:32.624 --> 00:02:34.778 According to the Ideal Gas Law, 00:02:34.802 --> 00:02:36.322 P-V N-R-T, 00:02:36.346 --> 00:02:38.823 pressure and temperature has a positive correlation, 00:02:38.847 --> 00:02:42.203 meaning they increase and decrease together. 00:02:42.227 --> 00:02:44.497 This is because the air expands in volume 00:02:44.521 --> 00:02:45.815 with less pressure, 00:02:45.839 --> 00:02:48.247 so the molecules have more room to wander around 00:02:48.271 --> 00:02:51.920 without colliding into each other and creating heat. 00:02:51.944 --> 00:02:54.087 Since the atmospheric pressure is a lot lower 00:02:54.111 --> 00:02:55.306 in high altitudes, 00:02:55.330 --> 00:02:56.866 it would be freezing cold 00:02:56.890 --> 00:02:58.644 if you were flying above the clouds. 00:02:58.668 --> 00:03:00.097 You'd need to wrap yourself up 00:03:00.121 --> 00:03:01.741 to keep your core body temperature 00:03:01.765 --> 00:03:03.566 above 95 degrees Fahrenheit, 00:03:03.590 --> 00:03:06.436 otherwise you'd start shivering violently, 00:03:06.460 --> 00:03:08.145 gradually becoming mentally confused 00:03:08.169 --> 00:03:09.789 and eventually drop out of the sky 00:03:09.813 --> 00:03:10.925 due to loss of muscle control 00:03:10.949 --> 00:03:13.179 from hypothermia! 00:03:14.272 --> 00:03:16.197 Now, the Ideal Gas Law implies 00:03:16.221 --> 00:03:17.903 that as the pressure decreases, 00:03:17.927 --> 00:03:19.732 gas volume increases. 00:03:19.756 --> 00:03:21.995 So, if you were to fly straight up too quickly, 00:03:22.019 --> 00:03:24.596 the inert gas in your body would rapidly expand 00:03:24.620 --> 00:03:27.433 the way soda fizzes up when shaken. 00:03:27.457 --> 00:03:30.008 The phenomenon is called "the bends," 00:03:30.032 --> 00:03:31.161 decompression sickness, 00:03:31.185 --> 00:03:32.873 or "divers disease" 00:03:32.897 --> 00:03:35.580 since deep sea scuba divers experience this 00:03:35.604 --> 00:03:37.523 when they come up too quickly. 00:03:37.547 --> 00:03:39.379 This results in pain, 00:03:39.403 --> 00:03:40.379 paralysis, 00:03:40.403 --> 00:03:41.606 or death, 00:03:41.630 --> 00:03:44.641 depending on how foamy your blood becomes. 00:03:44.665 --> 00:03:46.342 Okay, well, let's say you want to fly 00:03:46.366 --> 00:03:47.986 just a few meters above the ground 00:03:48.010 --> 00:03:49.820 where you can still see the road signs 00:03:49.844 --> 00:03:51.306 and breath oxygen with ease. 00:03:51.330 --> 00:03:53.346 You'll still need goggles and a helmet 00:03:53.370 --> 00:03:54.980 to protect you from birds, 00:03:55.004 --> 00:03:55.953 insects, 00:03:55.977 --> 00:03:56.757 street signs, 00:03:56.781 --> 00:03:57.693 electrical wires, 00:03:57.717 --> 00:03:59.550 and other flying humans, 00:03:59.574 --> 00:04:01.150 including flying cops 00:04:01.174 --> 00:04:02.319 ready to hand you a ticket 00:04:02.343 --> 00:04:04.528 if you don't follow the flying rules, buddy. 00:04:04.552 --> 00:04:07.025 Now remember, if you have a collision mid-air 00:04:07.049 --> 00:04:08.941 that knocks you unconscious, 00:04:08.965 --> 00:04:11.050 you would experience free fall 00:04:11.074 --> 00:04:12.933 until you hit the ground. 00:04:14.674 --> 00:04:17.551 Without society or the laws of physics, 00:04:17.575 --> 00:04:20.960 flying would be a totally awesome ability to have. 00:04:20.984 --> 00:04:22.775 But, even if we could all just float around 00:04:22.799 --> 00:04:23.757 a few feet above the ground 00:04:23.781 --> 00:04:25.465 and only moving at a snail's pace, 00:04:25.489 --> 00:04:28.992 I'm telling you, it's still a cool ability that I'd want, 00:04:29.016 --> 00:04:30.738 wouldn't you? 00:04:30.762 --> 00:04:32.892 Yeah, I thought so. 00:04:32.916 --> 00:04:35.392 Now, which superpower physics lesson 00:04:35.416 --> 00:04:37.779 will you explore next? 00:04:37.803 --> 00:04:42.043 Shifting body size and content, 00:04:42.067 --> 00:04:43.401 super speed, 00:04:43.425 --> 00:04:45.024 flight, 00:04:45.048 --> 00:04:47.972 super strength, 00:04:47.996 --> 00:04:50.658 immortality, 00:04:50.682 --> 00:04:52.216 and 00:04:52.240 --> 00:04:54.910 invisibility.