The sun， the moon， the planets， and other objects in space all have gravity. The pull of the sun is greater than that of the earth because the sun is so much bigger. The diameter of the sun is about 109 times the diameter of the earth. The gravity of the sun keeps the planets orbiting the sun. If there were no gravity pulling them toward the sun， they would spin off into space in a straight line. Scientists have learned about the laws of gravity and about the positions of the planets. They are now able to tell just where each planet is in space at any one time.
The laws of gravity were first discovered by Sir Isaac Newton. He said the idea came to him while he was in the garden drinking tea. An apple fell to the ground at his feet. He realized that the same force which drew the apple to the ground keeps the moon in its orbit around the earth.
While scientists know about the laws of gravity and how they work， they do not yet know or understand the cause of gravity. Why one body in the universe attracts another is still a mystery to them. The great mysterious forces which regulate the universe and the billions of galaxies out in space and keep them moving in perfect order can only be the work of our great God who created everything. He alone understands why the sun， the moon， the planets， and the billions and billions of galaxies move in such perfect order.
Vocabulary 词汇
mighty ["maiti] adj. 强有力的、强大的；
adv. 非常、很
diameter [dai"？mit？] n. 直径、放大率
straight [streit] adj. 直的、笔直的、不间断的、直率的；
adv. 直接地、径直地
orbit ["？：bit] n. 轨道；
vt. & vi. 在……轨道上运行、环绕轨道运行
mystery ["mist？ri] n. 神秘（性）、秘密（性）、神秘的事、来历不明的人
galaxy ["ɡ？l？ksi] n. 星系、银河系、一群（杰出或著名的人物）
Practice
Complete these sentences.
1．Gravity is the force， or pull， that keeps us from falling off the .
2．The pull of the sun is greater than that of the earth because the sun is than the earth．
3．The of the sun keeps the planets revolving around the sun．
4．The laws of gravity were first discovered by in the year 1687．
5．The force of gravity on the earth causes everything to be pulled toward the of the earth．
6．The diameter of the sun is about times that of the earth．
7．If there were no gravity pulling the planets toward the sun， they would move off into .
8．Scientists are able to tell where the planets are in at any one time．
Write true or false.
1．The earth is the only planet that has gravity．
2．The gravity of the sun keeps the planets orbiting the sun．
3．The laws of gravity were first discovered by Newton in 1927．
4．Scientists understand all about why one body in the universe attracts another one．
5．The idea of gravity came to Sir Isaac Newton while he was asleep．
6．Scientists still have much to learn about gravity．
7．Gravity causes things to have weight．
译文
地心引力定律是由艾萨克·牛顿在1687年发现的。他发现万物维系在一起和一直运动是由一种奇怪的力所驱使的。这种力就叫做重力，是一种能使物体相互靠近的一种自然力。尽管如此，在1687年之前，圣经上说是上帝用他强大的力量使整个世界连在一起的。
有些东西重量很轻，有些东西则很重。重力牵引着地球上的所有事物指向地心，由于重力的存在，你才能有重量，也因为有重力的存在，你才能生活在地球的表面而不是飞离地球。
在太空中的太阳、月亮和行星都有地心引力。因为太阳比地球大，太阳的牵引力比地球的牵引力大。太阳的直径是地球直径的109倍，太阳的地心引力使所有的行星都围绕着太阳旋转。如果没有太阳的地心引力牵引着其他行星，这些行星将会沿直线飞到其他太空中。科学家们已知道了地心引力定律和行星位置，但他们现在只能知道在某个任意时间行星在太空中的位置。
地心引力定律由艾萨克·牛顿首次发现。据牛顿说，他的灵感来自一次偶然的发现。当他在花园喝茶时，一个苹果从树上落下砸到了他的脚。他意识到就是让苹果落地的力使月亮在地球的轨道上围绕着地球旋转。
虽然科学家们知道了地心引力定律和它是如何作用的，但他们仍然不知道或者无法理解是什么产生了地心引力。他们也同样不能理解为什么在宇宙中的事物对另外一个事物有吸引。这种巨大的神秘力量控制着宇宙中的万物和太空中成千上万的银河系，并使它们有规律地飞行在太空中，而所有这些仅有创造万物的上帝能做到。也仅有上帝知道为什么太阳、月亮、行星和成千上万的银河系能如此有条理地、完美地飞行。
练习
完成以下句子。
1．重力是使我们不飞离 的压力或引力。
2．太阳的引力比地球的引力大，那是因为太阳比地球 。
3．太阳的 使行星围绕太阳旋转。
4．地心引力定律在1687年由 首次发现。
5．重力使地球上所有的事物都向地球的 靠拢。
6．太阳的直径是地球直径的 倍。
7．如果没有太阳的地心引力牵引着行星，它们都将脱离太阳到 。
8．科学家能说出在任何时间行星在 的位置。
判断正误。
1．地球是唯一一个有地心引力的行星。
2．太阳的地心引力使行星围绕着太阳旋转。
3．地心引力定律是在1927年首次由牛顿发现的。
4．科学家知道为什么宇宙中的一个物体能吸引着另外一个物体。
5．地心引力的灵感来源于艾萨克·牛顿的睡梦。
6．关于地心引力科学家们还有更多要学习的。
7．重力使物体具有重量。
完成句子：
1．earth 2．bigger 3．gravity
4．Isaac Newton 5．center 6．109
7．space 8．space
判断正误：
1．false 2．true 3．false
4．false 5．false 6．true
7．true
08 Magnetism 神奇的磁力
Man has used magnets and the forces that surround them to do many kinds of work. From your study of magnetism， you will remember that magnets are pieces of metal that have the power to attract other materials to themselves. Iron and steel are called magnetic materials because they are attracted to a magnet. Anything that has iron or steel in it can be attracted to a magnet.
Magnetism and electricity are closely related. A way of producing electricity has been discovered by using special magnets. If a magnet is moved near a wire， an electric current is made in the wire. If a wire is moved near a magnet， an electric current is also made.
In 1820 a Danish man named Hans Christian Oersted made a discovery. He discovered that a compass needle jumped when electric current was flowing through a wire held close to the compass. Oersted proved that electricity was affecting the magnet. After this discovery electromagnets were made. Electromagnets are used in many ways today. When an electromagnet is attached to a crane， it can be used as a lifting magnet. When the current flows through the coil， the crane can pick up a heavy load. When the current is broken， the load is dropped.
Small electromagnets are also used in many ways in homes， factories， and offices. An electric doorbell is an example of an electromagnet at work.
A machine that makes electricity in large amounts is called a generator. After power is generated in the power plant， it leaves the generator through wires. The wires connect with other wires leading to other parts of the country. Some wires are strung on huge poles； others are laid underground.
Vocabulary 词汇
magnet ["m？ɡnit] n. 磁铁、磁体、有吸引力的人（物）
steel [sti：l] n. 钢、钢铁；
vt. 使坚强、使坚定
compass ["k？mp？s] n. 罗盘、指南针、圆规、界限
needle ["ni：dl] n. 针、针状物
crane [krein] n. 鹤、起重机、吊车；
vt. & vi. 伸长、探头
coil [k？il] n. （一）卷、（一）圈、盘卷之物、线圈；
vt. & vi. 将……卷成圈或螺旋形
load [l？ud] n. 负荷、负担、装载、装载量、工作量、负荷量；
vt. & vi. 把……装上车
electromagnet [i"lektr？u"m？ɡnit] n. （物）电磁体、电磁铁
generator ["d？en？reit？] n. 发电机、发生器
Practice
Write true of false.
1．A generator is a machine that makes electric power．
2．Magnetism is a force of nature that can be seen easily．
3．Almost any type of material can be attracted by a magnet．
4．Electricity and magnetism are closely related．
5．Cranes can use electromagnets for lifting heavy loads．
6．Electricity can only travel on a closed or complete circuit．
译文
