crossing your nose’s path. But, if you could react quickly enough, do you even stand a

但如果你反應夠快，你有可能

chance to escape it’s smell? Could you outrun a fart?

逃過難聞的味道嗎？你逃得過屁嗎？

If you’re trying to outrun the sound...good luck! Sounds are vibrations that travel through

若你想要跑得比聲音還快...祝你好運！聲音是通過粒子震動

particles in a wave, and their speed is dependent on the medium they’re travelling in. For

所產生的聲波，而聲音的速度則跟傳導的媒介有關。例如，

example, sound travels 4x faster in water than air, because the particles are much closer

聲音在水中比在空氣中傳導快四倍，因為相較於空氣，水分子間的距離要近得多

together, meaning they more quickly transmit vibration energy. When travelling through

也就是傳導震動能量更迅速。當聲音在

air at sea level and 15℃, sound travels at 340 m/s - way faster than the fastest man

海拔0公尺、15℃的空氣中傳導時，速率是每秒340公尺，比地球上最快的男人

on earth, Usain Bolt, who can only move 12.4 m/s. Not even the fastest animal, the

閃電波特還要快多了，他每秒只能跑12.4公尺。就連世界最快的動物，

peregrine falcon which can travel 82 m/s - could soar past the sound of it’s own toot.

游隼，每秒能飛82公尺，也無法飛越自己的鳴叫聲。

But it’s the smell that most are concerned about escaping anyway, and here’s where

但多數人想躲過的仍是氣味本身，這也是

it gets a bit more complicated. Unlike sound waves and visible light which travel through

比較複雜的部份。與在空氣中傳遞的聲波和可見光不同的地方在於，

the air, smell is made up of the odour compounds that create the air. When you cut the cheese,

氣味是由氣味化合物組成的空氣。當你切開起司時，

odor molecules travel through the air and up your nose where they bind to odorant receptors.

氣味分子藉由空氣傳到你的鼻子，跟你的氣味接收器連結

And each individual has a unique set of receptors, meaning we all perceive smell differently.

而每個人的接收器都不同，代表我們聞到的其實都不盡相同。

At the basic level, smells travel through the process of diffusion whereby particles

一開始，氣味會經過一個擴散的階段，其中粒子

move from area of high concentration to low concentration, until reaching a state of equilibrium.

會由高濃度的地方移向低濃度的低方，直到均衡為止

But the composition of every fart is unique, based on the food you ate, your gut bacteria

但每次放屁的組成都會依據你吃的食物、內臟內負責分解

breaking it down, and the swallowed air you took in. Yup - farts are like snowflakes;

的細菌、還有吞進去的空氣而有所不同。是的，屁就像雪花一樣

each with it’s own variance. But the average fart tends to be composed of 59% nitrogen,

每個都有不同的樣子。但一般的屁通常都是由59%的氮氣、

21% hydrogen, 9% carbon dioxide, 7% methane, 3% oxygen, and only around 1% is made up of

21%的氫氣、9%的二氧化碳、7%的甲烷、3%氧氣所組成，至於僅剩的1%才是

the smelly stuff. Even knowing this, we would still need to know the concentration at the

難聞的東西。就算知道這些，我們還是得找出來源的濃度、

source, the temperature, air pressure, and wind movement in the room - which even in

氣溫、氣壓、還有房間內的氣流，就算只是在一個

a very still room is significant. Anecdotal evidence on the internet suggests that farts

毫無動靜房間裡這些條件也是很重要的。網路謠傳

are ejected at about 3 m/s, but there is little scientific evidence to back this up.

我們放屁的速度是每秒三公尺，但這乎沒什麼科學根據。

However, if we take into account the temperature of the gas, pressure and kinetic energy of

然而，若我們同時考量氣體的溫度、壓力、和分子的動能，

molecules, then the kinetic theory of gasses can help predict the average speed of a gas

那氣體動力學就能幫助我們預測氣體分子的

molecule. But since a fart is made up of many molecules, let’s just focus on one of the

平均速度。但由於一個屁是由很多分子組成的，所以先來看其中一個較難聞

smellier parts: skatole. If we use the kinetic theory of gasses, assume that the fart is

的分子吧：糞臭素。如果我們用氣體動力學假設這個屁

at body temperature, and then we plug in the molar mass of skatole, we calculate that the

跟體溫一樣，接著代入糞臭素的摩爾質量，算出

smelly skatole may be travelling around 243 m/s! Which is obviously faster than a human

這難聞的糞臭素以大約每秒243公尺的速度在移動！無庸置疑地，這速率遠遠超出一個人的移動速度。

can move.

這難聞的糞臭素以大約每秒243公尺的速度在移動！無庸置疑地，這速率遠遠超出一個人的移動速度。

Of course, these molecules aren’t travelling in an uninterrupted line but are constantly

當然，這些分子的移動並非完全不受干擾，且會經常

colliding into other air particles and physical matter. As a result the gas cloud as a whole

跟空氣中的其他粒子和物質碰撞。最後導致整團氣體

travels much slower still guided by diffusion. And it doesn’t take into account any air

以較慢的速度移動並擴散。而這尚未將

movement like wind or the ejection direction, which could either work for or against you.

氣流或是放屁的方向給考慮進去，而水能載舟亦能覆舟。

So, while you certainly can’t run faster than the absolute speed of smelly fart particles,

所以，雖然你不可能移動得比屁中臭粒子更快

with a little luck, you might be able avoid the fart itself!

幸運的話，你還是有可能逃過一劫的！

Want more fart science? Check out our videos “Are Silent Farts More Deadly?” and “Why

想知道更多關於放屁的科學？去看看我們的「無聲屁是否更臭？」和

Do We Like Our Own Farts?” By clicking the screen or using the link in the description.

「為什麼我們會喜歡自己放的屁？」這兩部影片吧！點擊螢幕或是影片簡介裡的連結

And subscribe for more weekly science videos!

來訂閱更多每週科學影片吧！