力学专业英语_4.pdf

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1、Lesson 5 Gravitation in the universe We are so used to our life on the surface of the earth that it can be quite an effort for our minds to break free of all the ideas that we take for granted.We talk about up and down ,but we know that what is down for us is up for someone else on the other side of

2、 the world.We can feel that things are heavy,and we often think of weight as being a fixed quality in an object,but it is not really fixed at all.If you could take a one-pound packet of butter 4,000 miles out from the earth,it would weigh only a quarter of a pound.Why would things weigh only a quart

3、er as much as they do at the surface of the earth if we took them 4,000 miles out into space?The reason is this:all objects have a natural attraction for all other objects,this is called gravitational attraction.But the power of attraction between tow objects gets weaker as they get farther apart.Wh

4、at about the weight of our one-pound packet of butter on the surface of the moon?On the moon there will be an attraction between the butter and the moon,but the butter will weigh only about one sixth as much as it does on the earth.This is because the moon is so much smaller than the earth.The amoun

5、t of gravitational pull that a body produces depends on the amount of material in it.So this is one of the first things we need to remember:that the weight of an object in space is not the same as its weight on the surface of the earth.Gravitation is a very important force in the universe.Every obje

6、ct has a gravitational pull,which is rather like magnetism.But,unlike magnetism,gravitation is not found only in iron and steel,it is in every object large or small.But large objects,such as the earth,have a stronger pull than small ones.Sir Isaac Newton,the great scientist of the seventeenth centur

7、y,first studied gravitation.When he was a boy,he often saw apples falling to the ground.He wondered why they fell towards the earth,and why they did not fly up into the sky.According to he law which he later produced,everything in the universe attracts everything else towards itself.The sun attracts

8、 the earth and the earth attracts the sun.The earth attracts the moon and the moon attracts the earth.Although the bigger object has the stronger attraction,all objects in fact have some.But we do not notice the gravitational pull of a piece of butter because the pull of the earth is very much great

9、er.Why does the earth always move round the sun,and not fly off into cold space?The suns gravitation gives the answer.The earth is always trying to move away in a straight line,but the sun is always pulling it back.So it continues on its journey round and round the sun.The earth is in a great group

10、of stars.We call this group the galaxy.The sun is one of the stars in the galaxy,in which there are about 100,000 million stars.It is not in the middle of the galaxy,but rather near one edge.These stars form a group,the shape of which is rather like the shape of a watch.Outside the galaxy there is e

11、mpty space,but thousands of millions of miles away there is another galaxy.Light from this other galaxy reaches us after about 2 million years.In fact,there are millions of these galaxies in the universe,and they appear to be rushing away from us.Many astronomers believe that there are thousands of

12、millions of millions of suns and some of there suns have planets as our sun does.The astronomers at Mount Palomar and Mount Wilson can see some of them well,but they cannot see one as it is now.The light takes millions of years to arrive here,so they see a distant galaxy as it used to be.The light l

13、eft it millions of years ago.It traveled across space and then went into an astronomers eye.Perhaps no men were living when it started.Gravitation is the force which holds all the atoms of a star together.It holds the sun together and it holds the atoms of the earth together.It holds us on the earth

14、.If there were no gravitation,we and everything else would fly off the earth into space.Einstein produced a new law of gravitation.Its main results are the same as the results of Newtons law.But in very small and fine matters,Einsteins law gives different results.One of there is that gravitation ben

15、ds light a little.But according to Newtons law gravitation has very little effect on light.Einstein showed this fact by means of mathematics,and not buy experiment.This result of his law was tested during an eclipse of the sun.Usually,when the light of a star passes the sun,we cannot see it,the sun

16、is too bright.But sometimes the earth moves between the sun and the moon.Then the earths shadow falls on the moon,no light from the sun can then reach the moon.The moon gets dark because it cannot reflect the suns light.We call this an eclipse of the moon.Sometimes the moon goes in front of the sun.

17、We can watch its edge when ti slowly crossed the suns disc.Everything gets darker and darker.Then,at last,we cannot see any part of the suns disc.The moon is hiding it completely.That is a total eclipse of the sun.Sometimes only part of the suns disc is hidden,that is not a total eclipse.It is a par

18、tial eclipse of the sun.During an eclipse of the sun,the suns light is shut out by the moon,then we can see the light of the star.A ray of light usually ravels in a straight line,but sometimes it bends.As shown in Fig.1,the astronomers who watched noticed that the star appeared to change its position a little.The cause of this was that the stars light was turning from the straight line as it passed the sun.The suns gravitation was bending the beam of light.This showed that Einstein was right.