萬物簡史 第67期:事物的測定(25)
裝配完畢以后，米歇爾的儀器看上去很像是一臺18世紀(jì)的鸚鵡螺牌舉重練習(xí)機(jī)。它由重物、砝碼、擺錘、軸和扭轉(zhuǎn)鋼絲組成。
At the heart of the machine were two 350-poundlead balls, which were suspended beside twosmaller spheres. The idea was to measure the gravitational deflection of the smaller spheresby the larger ones, which would allow the first measurement of the elusive force known as thegravitational constant, and from which the weight (strictly speaking, the mass) of the Earthcould be deduced.
儀器的核心是兩個(gè)635千克重的鉛球，懸在兩個(gè)較小球體的兩側(cè)。裝配這臺設(shè)備的目的是要測量兩個(gè)大球給小球造成的引力偏差。這將使首次測量一種難以捉摸的力--所謂的引力常數(shù)--成為可能，并由此推測地球的重量(嚴(yán)格來說是質(zhì)量)。
To a physicist, mass and weight are two quite different things. Your mass stays the samewherever you go, but your weight varies depending on how far you are from the center of someother massive object like a planet. Travel to the Moon and you will be much lighter but no lessmassive. On Earth, for all practical purposes, mass and weight are the same and so the termscan be treated as synonymous. at least outside the classroom.
對于物理學(xué)家而言，質(zhì)量和重量是兩個(gè)截然不同的概念。無論在哪兒，物體的質(zhì)量總是不變的，但是物體重量大小的變化取決于其與另一個(gè)如行星那樣的巨大物體的中心距離。在月球上我們會變得很輕，但是我們的質(zhì)量卻不變。在地球上，為了實(shí)用起見，質(zhì)量和重量數(shù)值上是相等的，因此可以用相近的辦法處理，至少在教室外可以這么做。
Because gravity holds planets in orbit and makes falling objects land with a bang, we tend tothink of it as a powerful force, but it is not really. It is only powerful in a kind of collectivesense, when one massive object, like the Sun, holds on to another massive object, like theEarth. At an elemental level gravity is extraordinarily unrobust. Each time you pick up a bookfrom a table or a dime from the floor you effortlessly overcome the combined gravitationalexertion of an entire planet. What Cavendish was trying to do was measure gravity at thisextremely featherweight level.
引力使行星保持在軌道上，使物體砰然墜落，因此很容易被認(rèn)為是一種強(qiáng)大的力，其實(shí)不然。它只是在整體意義上強(qiáng)大：一個(gè)巨大的物體，比如太陽，牽住另一個(gè)巨大的物體，比如地球。在基礎(chǔ)的層面上，引力極小。每次你從桌子上拿起一本書，或從地板上拾起一枚硬幣，你毫不費(fèi)勁就克服了整個(gè)行星施加的引力。卡文迪許想要做的，就是在極輕的層面上測量引力。
