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the gravitational force between the sun and the earth or any other graviational attractive force will be along the centres of the planets (radially) and do not contribute to any torque) as torque=r X F =rFsin@ but sin0=0 so radial forces cannot cause torque then how does the earth magically spin along its own axis?
a very striking question but most importantly it is so because when it was born, it was merely a lump of gases which were spinning very fast in random directions.. now as the gases condensed, the rotation speeds increased for the conservation of angular moment.... and now as the we have soldified, the the other combined forces of gravity from other heanely bodies is causing us to slow down. (albeit at a very very very slow rate). its a complex combined effect of many forces... the suns gravitational pull, and the moon's gravitational pull while the earth. check out the link below it has beautiful explanation (my knowledge comes from this source itself!!) http://www.physorg.com/news105637304.html
Hah! I was about to post the same link :P
The only other thing to note is that it keeps spinning because there are no appreciable forces to stop it.
that was really a cool convincng explanation which roots towards the big bang theorey and earth formation as a result
Yep. Actually, spin of a body is mostly an intrinsic(internal) property. It means that it is a rotation of a body about its own axis. If you go to space with a wheel, and then give that wheel a push(so as to give it a torque), it will spin forever and ever. Same thing happens with the earth, it is like a large sphere that has been taken into space. It was provided the torque during the formation of the solar system. The same explanation holds true for spin of an electron rotating inside an atom, around a nucleus. How do you think that no one is able to stop an electron's spin by applying so many forces on all forms of matter in daily life?
There can't be any torque causing the rotation of the earth, it is simply the conservation of angular momentum. Consider this definition of torque\[I \dot \omega = \tau\]If the days remains relatively the same length, then there is no angular acceleration \(\dot \omega\) and thus no torque. As mentioned by other posters, the earth was given it's spin during the creation of the galaxy.
...during the formation of the earth. (The universe itself is many billion years older.)