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How many times have you heard a jet plane going over, looked up to see it, but not been able tofind it even though you were looking toward where the sound was coming from?
有多少次你聽(tīng)到一架噴氣式飛機(jī)飛過(guò)，抬起頭去看它，即使你朝著聲音傳來(lái)的地方望去，卻仍然無(wú)法找到它？
Well, thismoment of science is going to tell you why you couldn't find it?
那么這次科學(xué)時(shí)刻將會(huì)告訴你為什么你找不到它。
Sometime during your life you might have been told that the plane wasn't with the sound becauseit was traveling faster than the speed of sound.
在你生活中的某個(gè)時(shí)候，你可能被告知看到的飛機(jī)和聽(tīng)到的聲音是不在同時(shí)出現(xiàn)的，這是因?yàn)楣獾膫鞑ニ俣缺嚷曇艨臁?/div>
It might have been, but it didn't have to be goingthat fast in order to appear to be separated from the sound.
它可能是這樣的，但它沒(méi)有必要為了同聲音分離而飛的那么快。
The reason the jet appeared to be apart from the noise it was making was simply that soundtravels a lot more slowly than light-about a million times more slowly.
噴氣式飛機(jī)之所以和它產(chǎn)生的噪音相分離，是因?yàn)槁曇魝鞑ニ俣缺裙馑俾话偃f(wàn)倍。
For quick review, at sea levelsound travels about seven-hundred-fifty miles per hour, light travels at 186,000 miles per second.
簡(jiǎn)單回顧一下，在海平面上，聲音的傳播速度大約是每小時(shí)750英里，而光速則是每秒186，000英里。
Commercial jets normally fly at about thirty-thousand feet.
商用噴氣式飛機(jī)一般在30，000英尺的高空飛行。
From that height it takes sound aboutthirty seconds to reach earth.
從那個(gè)高度，聲音傳送到地面需要30秒的時(shí)間，
Light gets here in only about four one-hundred-thousandths of asecond, or for practical purposes, instantaneously.
而光到達(dá)地面僅僅需要4/100，000秒，實(shí)際上就是瞬間到達(dá)。
A plane travelling at six-hundred miles an hour goes five miles in the thirty seconds it takes for thesound to reach you.
一架每小時(shí)飛行600英里的飛機(jī)在30秒中飛行了5英里，而聲音傳到你所在的地方需要30秒。
By the time the sound from the plane reaches your ears, the plane is fivemiles from where that sound originated.
在這30秒的時(shí)間里，聲音從飛機(jī)傳到你的耳朵里，而飛機(jī)離聲音發(fā)出的地方已經(jīng)有了5英里的距離。
So, when you look toward the sound, the plane is notthere anymore.
所以，當(dāng)你朝聲音發(fā)出的地方望去的時(shí)候，飛機(jī)早已不在那里了。
