为什么地球会自转???

jiko

原帖由 0123907395 于 17-9-2005 09:38 PM 发表
但我还是不清楚:自千年前开始地球就受过无数的庞大陨石撞击,但还是没有改变过地球自转的速度(还是一天有24小时)...

多了一秒了!
就是说如果要更新时钟的话是11:59 到11:60 然后才12:00

0123907395

原帖由 jiko 于 17-9-2005 10:42 PM 发表


多了一秒了!
就是说如果要更新时钟的话是11:59 到11:60 然后才12:00

什么意思?

sunny86

原帖由 jiko 于 17-9-2005 10:42 PM 发表


多了一秒了!
就是说如果要更新时钟的话是11:59 到11:60 然后才12:00

不是越转越慢吗 ?

RS25

原帖由 sunny86 于 18-9-2005 11:09 AM 发表

不是越转越慢吗 ?

对，肯定越来越慢。4亿年前，地球自转一周才21小时，一年有410天。过后越来越慢，到现在的每天24小时，一年365天。而且还以每百年慢0。0016秒的速度慢下来。

0123907395

原帖由 RS25 于 23-9-2005 12:26 PM 发表


对，肯定越来越慢。4亿年前，地球自转一周才21小时，一年有410天。过后越来越慢，到现在的每天24小时，一年365天。而且还以每百年慢0。0016秒的速度慢下来。

你是看书得知的吗?科学家怎么连四亿年前的时间也研究出来啊...

sade

Why do planets rotate?

Why do the planets rotate? What force cause them to rotate?

There is no force that causes the planets to rotate. Most of the rotation comes about from the conservation of angular momentum. Angular momentum is given by L=m*w*r2 where m is the mass, w is the angular velocity in rad/s and r is the radius of the circular motion. Due to conservation of angular momentum, if the radius of the orbit decreases, then its angular velocity must increase (as the mass is constant).

All planetary and stellar systems are born from the collapse of dense interstellar clouds. The clouds may originally be very large (even thousands of light years across). Consider a portion of the cloud the collapses from a size of a light year or so to the size of the solar system. That is a huge change in the size of the system. So, the very slight rotation that the cloud has in the beginning is increased dramatically when the collapse takes place. In fact, this is one of the barriers in star formation: there is excess angular momentum and there has to be a way of losing angular momentum before you can form a star.

Anyway, the bottom line is that stars like the Sun spin from the original angular momentum that was there in the solar nebula from which it formed. Not only that, all orbital motion of the planets (including the spin) is due to this orginal angular momentum.

You are saying that original angular momentum of the cloud causes orbital motions and rotations of the planets(mostly). But in the case of orbital motions we have gravitational force that gives us some restrictions of movement(Kepler laws,for example).

What I am saying is that there will be no planets if there was no initial angular momentum in the primordial solar nebula. If a nebula with absolutely no rotation collapses, then there will only be a central non-rotating star and there will not be any planets. Planets form out of a protostellar disk, which itself forms only because of the initial angular momentum of the cloud. The dynamics of a rotating body is of course controlled by forces like gravity. Kepler's laws are a direct consequence of gravity.

Are there some laws also in the case of rotations?

The only thing that has to be kept in mind in rotation is that it results in a centrifugal acceleration that points radially from the center of motion. Hence, there has to be some force that conteracts this acceleration; otherwise the body will fly away (in case of orbital motion) or will disintegrate (in case of spinning). In the case of orbital motion, the counteracting force is gravity; gravity causes the body to continually fall towards the center, and this exactly conteracts the force resulting from the centripetal acceleration. In the case of a spinning object, it is the self-adhesion of the body itself that keeps it together. This results in a limit for how fast an object can rotate and still keep itself together. If it rotates too fast, the outward acceleration felt by the elements in the body may be more than the force that keeps them bonded together, and if this happens, the body breaks up. Other than this, there is no real law concerning rotations. (Note that rotational motion involves conservation of angular momentum just like linear motion conserves linear momentum).

好像很复杂，不想翻译了。。

RS25

原帖由 0123907395 于 24-9-2005 09:21 AM 发表




你是看书得知的吗?科学家怎么连四亿年前的时间也研究出来啊...

科学家是根据远古时代贝类化石而得知的。4亿年前贝类壳上的条纹有4百多条。现在贝类的条纹有365。

0123907395

原帖由 RS25 于 24-9-2005 01:47 PM 发表


科学家是根据远古时代贝类化石而得知的。4亿年前贝类壳上的条纹有4百多条。现在贝类的条纹有365。

那他们如何知道以前的一天超过24小时?

jiko

原帖由 sunny86 于 18-9-2005 11:09 AM 发表

不是越转越慢吗 ?

那也许是我看错了!
哈哈