WEBVTT 00:00:00.000 --> 00:00:08.766 (Machine translation by DeepL translate)We've been experimenting with gravity that is 10 times stronger than the Earth's (=10G). 00:00:09.500 --> 00:00:16.433 I wonder how much gravity we can actually withstand. 00:00:16.966 --> 00:00:21.866 Gravity (strong) 00:00:21.866 --> 00:00:25.858 Gravity (Stronger) 00:00:31.633 --> 00:00:37.900 First, let's examine the strength of bones. 00:00:40.466 --> 00:00:45.966 Let's assume that the entire body weight is supported by a single leg. 00:00:46.919 --> 00:00:53.152 The tibia bone has a cross-sectional area of 1 cm2 and can withstand up to 1730 kg. 00:00:56.052 --> 00:01:02.319 Even if the gravity increases 10 times, the bone will not break. 00:01:04.233 --> 00:01:11.233 If the gravity becomes strong enough for a 50kg person to reach 4680kg 00:01:11.833 --> 00:01:19.066 In other words, if gravity becomes 94G, your shins will shatter under your own weight. 00:01:24.500 --> 00:01:30.500 This is for a stationary person. 00:01:31.900 --> 00:01:37.900 But if you run, the stress on your bones will only be doubled at most 00:01:39.194 --> 00:01:46.628 If you don't get knocked down, the problem is weakness of the muscles, not fractures. 00:01:48.333 --> 00:01:55.100 Let's think about standing up from a sitting position. 00:01:56.266 --> 00:02:04.600 You use your knee joint as a fulcrum to lift yourself up with your quadriceps muscles. 00:02:05.366 --> 00:02:18.266 The magnitude of the torque "r1×F" (×2) must be greater than the torque "r2×Mg" that gravity pulls the body to stand. 00:02:18.965 --> 00:02:28.365 If we substitute the average value, gravity must be less than 1.1G 00:02:29.200 --> 00:02:35.800 In other words, the earth's gravity is the limit for standing by muscle strength alone. 00:02:43.233 --> 00:02:49.566 Force F is proportional to the cross-sectional area of the muscle, so if you stand up at 10G 00:02:51.433 --> 00:02:58.100 the cross-sectional area must be at least 9 times larger. 00:03:07.866 --> 00:03:17.600 In reality, you can't train that much, and even a top athlete can only lift four times the weight of an average person in a squat 00:03:18.733 --> 00:03:26.966 Also, if we take into account the fact that you have to bend over, and if the gravity you can withstand is increased by 20%. 00:03:28.000 --> 00:03:35.500 Realistically, the limit for a trained person to stand up would be 5.3G. 00:03:36.666 --> 00:03:42.700 Next is the limit of gravity when walking. 00:03:43.700 --> 00:03:53.100 Let's consider the following model of walking 00:03:53.733 --> 00:04:01.466 An inverted pendulum moves forward with the feet as its fulcrum. 00:04:03.433 --> 00:04:11.600 The center of gravity moves up and down. The pendulum moves efficiently by exchanging kinetic energy and potential energy. 00:04:13.466 --> 00:04:21.800 Here, we refer to the world record of a powerful Icelandic man (179 kg). He can walk 5 steps with a 649kg log on his back. 00:04:22.666 --> 00:04:31.600 This is the limit of a person's strength, calculated by a walking model. 00:04:32.533 --> 00:04:42.700 When you don't carry a log, you know what the limit of gravity is. 00:04:43.666 --> 00:04:56.033 If you increase the gravity, you will not be able to walk at 4.6G 00:04:56.833 --> 00:05:04.266 Reference: The world record for weightlifting 62kg class shows that 4.0G is the limit. 00:05:04.266 --> 00:05:13.966 The limits of humanity as we know it 00:05:14.933 --> 00:05:23.133 Other than that, it is said that if you exceed the 5G experienced by fighter pilots, you will faint because not enough blood can be pumped to your head. 00:05:24.566 --> 00:05:33.066 "If we assume that it is safe to go below 5G", then out of the 594 exoplanets with known gravitational acceleration, 00:05:34.190 --> 00:05:44.390 469 planets seem to be safe for human bones and muscles. But some planets will break your bones when you arrive.