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We know that every planet in our solar system revolve's around the sun in a particular orbit. But were to they get the energy to revolve around the sun. And why do they not drop into the sun there is only gravitational force acting which is always attractive in nature?

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BTW, electrons don't fall into nuclei because of their own angular momentum, etc. considerations. If anything, the (weak) nuclear force helps them fall in - this is known as electron capture –  Chris White Mar 8 '13 at 15:16
    
    
to make it clear, one of he basic reasons for inventing quantum mechanics was that the electrons do not fall into the nucleus but exist in stable orbits. –  anna v Mar 8 '13 at 15:25
    
Related: physics.stackexchange.com/questions/54874/… –  dmckee Mar 8 '13 at 17:10
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They are technically falling to the sun. The gravitational force of the sun is what is keeping them in orbit around the sun and not floating away. But they are also moving really fast. They are moving so fast that the direction in which they are attracted to the sun is changing constantly and it makes them spin around it instead of actually falling into it.

And since they do not encounter large amounts friction while moving though space (It's vacum) they do not need energy to keep moving.

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