If humans could fly, without tools and machines, how fast do you think we would go?
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？
As of 2012, the world record for fastest short-distance sprint speed is roughly 27 miles per hour.
截止到2012年，世界短跑最快纪录是大约每小时27英里。
Running speed depends on how much force is exerted by the runner's legs,
跑步的速度取决于跑步者的腿部力量，
and according to Newton's Second Law of Motion, force is the product of mass times acceleration.
根据牛顿第二运动定律，力的大小等于质量乘以加速度。
And Newton's Third Law states that for every action, there is an equal and opposite reaction.
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。
So, that means running requires having a ground to push off from, and the ground pushes back against the runner's foot.
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。
So, flying would actually be more similar to swimming.
所以，飞行事实上更类似于游泳。
Michael Phelps is currently the fastest human in water and the most decorated Olympian of all time.
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。
Guess how fast he swims? The answer may surprise you.
猜猜看，他能游多快？答案可能让你不敢相信。
His fastest recorded speed is less than 5 miles per hour.
他最快的速度记录是小于每小时5英里。
A child on the ground can easily outrun Michael Phelps in water, but why is that?
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，但这是为什么呢？
Well, let's go back to Newton's Third Law of Motion.
好吧，让我们回到牛顿第三运动定律。
When we run, we move forward by pushing against the ground with our feet and the ground pushes back, propelling us forward.
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。
The ground is solid. By definition, it means the particles are essentially locked into place
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，
and must push back instead of getting out of the way, but water is liquid and flows easily.
所以只能推回去，而不是被推走。但是水是液体，易流动。
When we move our limbs to push back against the water,
当我们移动肢体来排开水的时候，
a part of the water molecules can just slide past one another instead of pushing back.
每一个水分子只会彼此滑过，而不是推回来。
Now, let's think about flying.
现在，让我们来想一想飞行。
Air has a lot more free space for particles to move past one another, so even more of our energy would be wasted.
空气中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。
We would need to push a lot of air backwards in order to move forward.
我们需要向后推大量的空气才能向前移动。
Astronauts move around in shuttles in zero gravity when they're in outer space
宇航员在航天飞机中，在外层空间的零重力情况下，
by pulling on handles installed on the ceiling walls and floors of the shuttle.
能通过拉动安装在航天飞机天花板和地板上的手柄移动。
Now, imagine you were given the ability to float. How would you move around in the middle of the street?
现在，想象你有浮动的能力。你会如何在街上移动？
Well, you wouldn't get very far by swimming in air, would you? Nah, I don't think so!
好吧，在空气中游泳你不会走很远，是吧？不，我不这么认为！

Now, assuming you were granted the ability to float and the speed to move around efficiently,
现在，假设你有漂浮的能力和高效的移动速度，
let's discuss the height of your flight. According to the Ideal Gas Law, P-V N-R-T,
让我们来讨论一下你飞行的高度。根据理想气体定律，P-V N-R-T
pressure and temperature has a positive correlation, meaning they increase and decrease together.
压强和温度呈正相关，这意味着它们同时增加或减少。
This is because the air expands in volume with less pressure,
这是因为减小压强，可以使空气体积膨胀，
so the molecules have more room to wander around without colliding into each other and creating heat.
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。
Since the atmospheric pressure is a lot lower in high altitudes, it would be freezing cold if you were flying above the clouds.
在高海拔地区，由于大气压力很小，如果你在云层上面飞行，气温会降低。
You'd need to wrap yourself up to keep your core body temperature above 95 degrees Fahrenheit,
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，
otherwise you'd start shivering violently, gradually becoming mentally confused
否则你就会开始猛烈地颤抖，逐渐变得神智恍惚，
and eventually drop out of the sky due to loss of muscle control from hypothermia!
并最终从天空坠落，无法控制肌肉因为低温，无法控制肌肉！
Now, the Ideal Gas Law implies that as the pressure decreases, gas volume increases.
现在，理想气体定律暗示，随着压力减小，气体体积增大。
So, if you were to fly straight up too quickly,
所以，如果你垂直向上飞得太快了，
the inert gas in your body would rapidly expand the way soda fizzes up when shaken.
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。
The phenomenon is called "the bends," decompression sickness, or "divers disease"
这种现象被称为the bens，减压病或潜水员病，
since deep sea scuba divers experience this when they come up too quickly.
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。
This results in pain, paralysis, or death, depending on how foamy your blood becomes.
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。
Okay, well, let's say you want to fly just a few meters above the ground
好吧，让我们说说你想要在仅高于地面数米飞行，
where you can still see the road signs and breath oxygen with ease.
你还可以看到道路标志，并轻松地呼吸氧气。
You'll still need goggles and a helmet to protect you from birds, insects, street signs, electrical wires, and other flying humans,
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，
including flying cops ready to hand you a ticket if you don't follow the flying rules, buddy.
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。
Now remember, if you have a collision mid-air that knocks you unconscious, you would experience free fall until you hit the ground.
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。
Without society or the laws of physics, flying would be a totally awesome ability to have.
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。
But, even if we could all just float around a few feet above the ground and only moving at a snail's pace,
但是，即使我们可能都只是离地面几英尺高处飘浮，并仅以蜗牛的速度移动，
I'm telling you, it's still a cool ability that I'd want, wouldn't you? Yeah, I thought so.
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。
Now, which superpower physics lesson will you explore next?
接下来，你打算去研究哪门超能力物理课？
Shifting body size and content, super speed, flight, super strength, immortality, and invisibility.
是改变人体大小和成分，超级快的速度，飞行，神力，长生不老还是隐身。