【科学】If humans could fly, without tools and machines, how fast do you think we would go?

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原文链接:https://www.ted.com/talks/joy_lin_if_superpowers_were_real_flight/transcript#t-63727

As of 2012, the world record for fastest short-distance sprint speed is roughly 27 miles per hour. Running speed depends on how much force is exerted by the runner’s legs, and according to Newton’s Second Law of Motion, force is the product of mass times acceleration. And Newton’s Third Law states that for every action, there is an equal and opposite reaction. So, that means running requires having a ground to push off from, and the ground pushes back against the runner’s foot. So, flying would actually be more similar to swimming. Michael Phelps is currently the fastest human in water and the most decorated Olympian of all time. Guess how fast he swims? The answer may surprise you. His fastest recorded speed is less than 5 miles per hour. A child on the ground can easily outrun Michael Phelps in water, but why is that? Well, let’s go back to Newton’s Third Law of Motion. When we run, we move forward by pushing against the ground with our feet and the ground pushes back, propelling us forward. The ground is solid. By definition, it means the particles are essentially locked into place and must push back instead of getting out of the way, but water is liquid and flows easily. When we move our limbs to push back against the water, a part of the water molecules can just slide past one another instead of pushing back. Now, let’s think about flying. Air has a lot more free space for particles to move past one another, so even more of our energy would be wasted. We would need to push a lot of air backwards in order to move forward. Astronauts move around in shuttles in zero gravity when they’re in outer space by pulling on handles installed on the ceiling walls and floors of the shuttle. Now, imagine you were given the ability to float. How would you move around in the middle of the street? Well, you wouldn’t get very far by swimming in air, would you? Nah, I don’t think so! Now, assuming you were granted the ability to float and the speed to move around efficiently, let’s discuss the height of your flight. According to the Ideal Gas Law, P-V N-R-T, pressure and temperature has a positive correlation, meaning they increase and decrease together. This is because the air expands in volume with less pressure, so the molecules have more room to wander around without colliding into each other and creating heat. Since the atmospheric pressure is a lot lower in high altitudes, it would be freezing cold if you were flying above the clouds. You’d need to wrap yourself up to keep your core body temperature above 95 degrees Fahrenheit, otherwise you’d start shivering violently, gradually becoming mentally confused and eventually drop out of the sky due to loss of muscle control from hypothermia! Now, the Ideal Gas Law implies that as the pressure decreases, gas volume increases. So, if you were to fly straight up too quickly, the inert gas in your body would rapidly expand the way soda fizzes up when shaken. The phenomenon is called “the bends,” decompression sickness, or “divers disease” since deep sea scuba divers experience this when they come up too quickly. This results in pain, paralysis, or death, depending on how foamy your blood becomes. Okay, well, let’s say you want to fly just a few meters above the ground where you can still see the road signs and breath oxygen with ease. You’ll still need goggles and a helmet to protect you from birds, insects, street signs, electrical wires, and other flying humans, including flying cops ready to hand you a ticket if you don’t follow the flying rules, buddy. Now remember, if you have a collision mid-air that knocks you unconscious, you would experience free fall until you hit the ground. Without society or the laws of physics, flying would be a totally awesome ability to have. But, even if we could all just float around a few feet above the ground and only moving at a snail’s pace, I’m telling you, it’s still a cool ability that I’d want, wouldn’t you? Yeah, I thought so. 

如果人类可以飞翔 而不是利用工具或者机器, 你觉得我们能飞多快? 截止到2012年,世界 短跑最快纪录 是大约每小时27英里(约43km/h)。 跑步的速度取决于 跑步者的腿部力量。 根据牛顿第二运动定律, 力的大小等于质量乘以加速度。 牛顿第三定律又说 每一个作用力 都有一个大小相等、方向相反的反作用力。 所以,这就意味着,跑步需要 有一个地面来作支撑, 地面通过反作用力把跑步者的脚推出去。 所以,飞行事实上 更类似于游泳。 迈克尔·菲尔普斯是目前水里游得最快的人, 也是有史以来最杰出的的奥林匹克运动员。 猜猜看,他能游多快? 答案可能让你不敢相信。 他最快的速度记录是 小于每小时5英里(8km/h)。 一个在地上奔跑的小孩能够很容易超过 在水里游泳的菲尔普斯。 但是,这是为什么呢? 好吧,让我们回到牛顿第三运动定律。 当我们跑步时,我们向前移动, 利用我们的脚去推地面 同时,地面反过来 推动我们向前。 地面是固体。 根据定义,物质颗粒 基本上被固定在原来的地方, 所以只能推回去,而不是被推走。 但是水是液体,易流动。 当我们移动肢体 来排开水的时候, 每一个水分子 只会彼此滑过 而不是推回来。 现在,让我们来想一想飞行。 空气种有更多的空间 让粒子进行自由移动, 所以会浪费我们更多的能量。 我们需要向后推大量的空气 才能向前移动。 宇航员在航天飞机中, 在外层空间的零重力情况下, 能通过拉动安装在航天飞机 天花板和地板上的手柄移动。 现在,想象你有浮动的能力。 你会如何在街上移动? 好吧,在空气中游泳你不会走很远 你不会走很远,是吧? 不,我不这么认为! 现在,假设你有漂浮的能力 和高效的移动速度, 让我们来讨论一下你飞行的高度。 根据理想气体定律, PV=NRT (P理想气体压强,V理想气体体积,N气体物质的量,R理想气体常数,T理想气体的热力学温度) 压强和温度呈正相关, 这意味着它们同时增加或减少。 这是因为减小压强,可以使 空气体积膨胀, 因此,空气分子有更多的空间来四处游荡 不会互相碰撞产生热量。 在高海拔地区, 由于大气压力很小, 气温会降低。 如果你在云层上面飞行, 你需要把自己包得严严实实 来保持你的正常体温 华氏 95 度以上(35摄氏度) 否则你就会开始猛烈地颤抖, 逐渐变得神智恍惚, 并最终从天空坠落, 无法控制肌肉因为低温 无法控制肌肉 ! 现在,理想气体定律暗示 随着压力减小, 气体体积增大。 所以,如果你垂直向上飞得太快了, 在你的身体中的惰性气体将迅速膨胀, 就像苏打水被摇了一摇。 这种现象被称为”the bens” 减压病, 或”潜水员病”, 因为深海潜水员浮出水面的速度过快时, 也会出现这种情况。 这会导致疼痛、 麻痹、 甚至死亡, 这取决于你血液的泡沫化程度。 好吧,让我们说说你想要在飞行 仅高于地面数米飞行, 你还可以看到道路标志, 并轻松地呼吸氧气。 你仍然需要护目镜和头盔 保护你免受鸟、 昆虫、 路牌、 电线、 和其他飞行的人的撞击, 包括飞行的警察 准备递给你一张罚单, 理由是你不遵守飞行规则,伙计。 现在,记住,如果你在半空中发生碰撞, 足以把你撞昏, 你会体验自由落体 直到你落到地面。 没有社会或物理定律, 飞行会成为一种完全令人敬畏的能力。 但是,即使我们可能都只是四处飘浮 离地面几英尺高, 仅以蜗牛的速度移动 我告诉你,它仍然是一种我想要的很酷的能力, 不是吗? 我就是这样认为的。