时间简史02｜空间和时间（中英文）仪表堂堂
CHAPTER 2SPACE AND TIME
第二章 空间和时间
Our prent ideas about the motion of bodies date back to Galileo and Newton. Before them people believedAristotle, who said that the natural state of a body was to be at rest and that it moved only if driven by a force orimpul. It followed that a heavy body should fall faster than a light one, becau it would have a greater pulltoward the earth.
ret是什么意思我们现在关于物体运动的观念来自于伽利略和牛顿。在他们之前，人们相信亚里士多德，他说物体的自然状态是静止的，并且只在受到力或冲击作用时才运动。这样，重的物体比轻的物体下落得更快，因为它受到更大的力将其拉向地球。
The Aristotelian tradition also held that one could work out all the laws that govern the univer by purethought: it was not necessary to check by obrvation. So no one until Galileo bothered to e whether bodiesof different weight did in fact fall at different speeds.
It is said that Galileo demonstrated that Aristotle’s beliefwas fal by dropping weights from the leaning tower of Pisa. The story is almost certainly untrue, but Galileodid do something equivalent: he rolled balls of different weights down a smooth slope. The situation is similar tothat of heavy bodies falling vertically, but it is easier to obrve becau the Speeds are smaller. Galileo’smeasurements indicated that each body incread its speed at the same rate, no matter what its weight. Forexample, if you let go of a ball on a slope that drops by one meter for every ten meters you go along, the ballwill be traveling down the slope at a speed of about one meter per cond after one cond, two meters percond after two conds, and so on, however heavy the ball. Of cour a lead weight would fall faster than afeather, but that is only becau a feather is slowed down by air resistance. If one drops two bodies that don’thave much air resistance, such as two different lead weights, they fall at the same rate. On the moon, wherethere is no air to slow things down, the astronaut David R. Scott performed the feather and lead weightexperiment and found that indeed they did hit the ground at the same time.
保证金英语亚里士多德的传统观点还以为，人们用纯粹思维可以找出制约宇宙的定律：不必要用观测
去检验它。所以，伽利略是第一个想看看不同重量的物体是否确实以不同速度下落的人。据说，伽利略从比萨斜塔上将重物落下，从而证明了亚里士多德的信念是错的。这故事几乎不可能是真的，但是伽利略的确做了一些等同的事——将不同重量的球从光滑的斜面上滚下。这情况类似于重物的垂直下落，只是因为速度小而更容易观察而已。伽利略的测量指出，不管物体的重量是多少，其速度增加的速率是一样的。例如，在一个沿水平方向每走10米即下降1米的斜面上，你释放一个球，则1秒钟后球的速度为每秒1米，2秒钟后为每秒2米等等，而不管这个球有多重。当然，一个铅锤比一片羽毛下落得更快，那是因为空气对羽毛的阻力引起的。如果一个人释放两个不遭受任何空气阻力的物体，例如两个不同的铅锤，它们则以同样速度下降。航天员大卫·斯各特在月亮上进行了羽毛和铅锤实验。因为没有空气阻碍东西下落。
Galileo’s measurements were ud by Newton as the basis of his laws of motion. In Galileo’s experiments, as abody rolled down the slope it was always acted on by the same force (its weight), and the effect was to make itconstantly speed up. This showed that the real effect of a force is always to change the speed of a body, ratherthan just to t it moving, as was previously thought. It also meant that whenever a body is not acted
pentahoon by anyforce, it will keep on moving in a straight line at the same speed. This idea was first stated explicitly in Newton’sPrincipia Mathematica, published in 1687, and is known as Newton’s first law. What happens to a body when aforce does act on it is given by Newton’s cond law. This states that the body will accelerate, or change itsspeed, at a rate that is proportional to the force. (For example, the acceleration is twice as great if the force istwice as great.) The acceleration is also smaller the greater the mass (or quantity of matter) of the body. (Thesame force acting on a body of twice the mass will produce half the acceleration.) A familiar example isprovided by a car: the more powerful the engine, the greater the acceleration, but the heavier the car, thesmaller the acceleration for the same engine.
carena
伽利略的测量被牛顿用来作为他的运动定律的基础。在伽利略的实验中，当物体从斜坡上滚下时，它一直受到不变的外力（它的重量），其效应是它被恒定地加速。这表明，力的真正效应总是改变物体的速度，而不是像原先想像的那样，仅仅使之运动。同时，它还意味着，只要一个物体没有受到外力，它就会以同样的速度保持直线运动。这个思想是第一次被牛顿在1687年出版的《数学原理》一书中明白地叙述出来，并被称为牛顿第一定律。
物体受力时发生的现象则由牛顿第二定律所给出：物体被加速或改变其速度，其改变率与所受外力成比例。（例如，如果力加倍，则加速度也将加倍。）物体的质量（或物质的量）越大，则加速度越小，（以同样的力作用于具有两倍质量的物体则只产生一半的加速度。）小汽车可提供一个熟知的例子，发动机的功率越大，则加速度越大，但是小汽车越重，则对同样的发动机加速度越小。
In addition to his laws of motion, Newton discovered a law todescribe the force of gravity, which states that every body attracts every other body with a force that isproportional to the mass of each body. Thus the force between two bodies would be twice as strong if one ofthe bodies (say, body A) had its mass doubled. This is what you might expect becau one could think of thenew body A as being made of two bodies with the original mass. Each would attract body B with the originalforce. Thus the total force between A and B would be twice the original force. And if, say, one of the bodies hadtwice the mass, and the other had three times the mass, then the force would be six times as strong. One cannow e why all bodies fall at the same rate: a body of twice the weight will have twice the force of gravitypulling it down, but it will also have twice the mass. According to 彼岸花的英文
morningcallNewton’s cond law, the two effects willexactly cancel each other, so the acceleration will be the same in all cas.
日语翻译公司>gprs是什么意思
除了他的运动定律，牛顿还发现了描述引力的定律：任何两个物体都相互吸引，其引力大小与每个物体的质量成正比。这样，如果其中一个物体（例如A）的质量加倍，则两个物体之间的引力加倍。这是你能预料得到的，因为新的物体A可看成两个具有原先质量的物体，每一个用原先的力来吸引物体B，所以A和B之间的总力加倍。其中一个物体质量大到原先的2倍，另一物体大到3倍，则引力就大到6倍。现在人们可以看到，何以落体总以同样的速率下降：具有2倍重量的物体受到将其拉下的2倍的引力，但它的质量也大到两倍。按照牛顿第二定律，这两个效应刚好互相抵消，所以在所有情形下加速度是同样的。
cancer是什么意思中文
Newton’s law of gravity also tells us that the farther apart the bodies, the smaller the force. Newton’s law ofgravity says that the gravitational attraction of a star is exactly one quarter that of a similar star at half thedistance. This law predicts the orbits of the earth, the moon, and the planets with great accuracy. If the lawwere that the gravitational attraction of a star went down faster or incread more rapidly with distance, theorbits of t
he planets would not be elliptical, they would either spiral in to the sun or escape from the sun.
