If humans could fly, without tools and machines, how fast do you think we would go? 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. Now, which superpower physics lesson will you explore next? Shifting body size and content, super speed, flight, super strength, immortality, and invisibility.
如果人類能在不依靠任何工具 或機器的情況下飛行 你認為我們能飛多快? 2012 年的世界紀錄中 短跑最快的 大約時速 43 公里(27 英哩) 跑步的速度視跑者的腿 能發揮多少力量 根據牛頓第二運動定律 力量是物體質量乘上加速度 牛頓第三定律則顯示 每個動作 都會有相同的反作用力 所以這表示跑步需要 地面讓跑者施力 也以同樣的力量推向跑者的腳 飛行實際上 會比較像游泳 Michael Phelps(美國奧運游泳選手) 是目前游泳最快的人 也獲得最多奧運金牌 你猜他能游多快? 答案可能讓你很驚訝 他最快的記錄 時速低於 8 公里(5 英哩) 一個在地面跑步的小孩 都可以輕易超過 Michael Phelps 在水中前進的速度 但為何會這樣呢? 讓我們再回到牛頓的第三運動定律 當我們跑步時 藉由雙足推向地面的力量前進 地面也回施力量 推動我們前進 地面是固體 顯然地,這表示 粒子的本質是固著不動的 所以一定會把力量推回,而不是被推開 但水是液體,易於流動 當我們移動四肢 去推動水的時候 部分的水分子 就會滑開 並不會把力回推 現在我們來思考關於飛行 空氣中有更多的空間 能讓分子移動 所以我們就會耗掉更多的能量 我們得要把很多的空氣向後推 才能前進 宇航員在太空梭裡 外太空無重力的狀態下 他們要拉住安裝在牆上或地板上的把手 才能移動 現在想像你能漂浮 你要怎樣在路中間移動呢? 光靠著在空氣中游泳 你沒辦法前進多少的,不是嗎? 喔,我可不認為你能有多快 現在假設你能漂浮 也能很有效率的四處移動 那讓我們來討論飛行的高度 根據理想氣體方程式(Ideal Gas Law) 壓力 (P)、體積 (V)、莫爾數 (n) 理想氣體常數 (R) 與絕對溫度 (T) 壓力和溫度是正相關 表示它們會同時升高或降低 這是因為空氣在較小的壓力下 體積會膨脹 所以分子能有更多空間游移 而不會彼此碰撞產生熱能 既然在高空中 大氣壓力低得多 溫度會變得很低 你要在雲上飛行的話 你得把自己包得緊緊的 才能讓體溫 不低於攝氏 35 度(華氏 95 度) 否則你會開始不停的發抖 然後神智不清 最後因為低溫造成 肌肉失控 從天上掉下來 依照理想氣體方程式(Ideal Gas Law) 當壓力降低 氣體的體積會增加 所以如果你一下子飛太高 你體內的惰性氣體會急速擴大 就像汽水被搖過之後嘶嘶作響 這現象叫做 「減壓症」(the bends) 因為減壓造成的疾病 又叫「潛水夫病」(divers disease) 用水肺游到深海的潛水伕 如果太快浮上水面 就會經歷相同的情況 這會造成疼痛 癱瘓 甚至死亡 端看你體內血液中氣泡量有多少 好吧,那我們假設你只想 在離地幾公尺的地方飛行 在你還能看到道路標示的高度 也能輕鬆的呼吸到氧氣 你還是需要風鏡和安全帽保護 因為會有鳥類 昆蟲 街道標示 電線 還有其他飛行中的人們 包括飛行警察 等著要開你罰單 如果你沒遵守飛行規則的話 要記得,如果你在半空中碰撞 導致昏迷 你會變成自由落體 直到撞上地面 如果沒有社會規範或是物理定律 那飛行真的是種超棒的能力 但是,即使我們只能在 離地幾公尺的地方漂浮 用蝸牛般的速度前進 我跟你說,我依然想 擁有這種很酷的能力 你也是吧? 沒錯,我也這麼想 接下來,你想探索 哪個超能力物理學課程? 改變身體大小和成分? 超高速? 飛行能力? 超級大力士? 永生不死? 或是 隱形力?