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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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marked as duplicate by Rob Jeffries, Kyle Kanos, Brandon Enright, Danu, Qmechanic Dec 17 '14 at 17:34

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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/q/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 a near-vacuum) they do not need energy to keep moving.

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Let me answer another component: where the initial energy for their movement came from.

Imagine two bodies separated by a large distance. In this case, the gravitational pull is small and the gravitational potential is low. Their relative velocities are just about zero. For all intensive purposes, our energy accounting is zeroed out. KE=0 GE=0 (kinetic and gravitational).

This isn't a bad description of the cloud of gases from which our solar system formed. True, the atoms which will later make up the planets and sun were mixed and dispersed, but the above statements about kinetic and gravitational energy still roughly applied.

The energy needed to start an orbit then came from putting gravitational potential into the negatives. This is why gravitational potential is GE=-G m1 m2 / r. We had zero energy to start with, and we end with gravitational potential energy being -2 units.

Why 2? Because closing of the distance between the bodies liberated energy which went equally into 2 different places. One is kinetic, which is currently manifested as such in the orbit, the other is frictional losses. This went to heat things up, and then was radiated into space. I'll call this thermal energy liberated from the gravitational change TE.

Beginning state:

$$ GE + KE + TE = 0 \\ 0 + 0 + 0 = 0 $$

Final State:

$$ GE + KE + TE = 0 \\ -2 + 1 + 1 = 0 $$

Total energy is constant, satisfying conservation of energy.

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According to some theories (like Big Bang) the planets are parts of greater bulks of matter as they gradually collided and separated (after becoming cold). They have gotten a speed due to the explosion and then they got stuck in the gravitational field of bigger planets and stars. As they rotate in the vacuum around them, they don't lose their velocity due to some frictional mechanisms.

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The Big Bang has nothing to do with the planets. This makes no sense. –  HDE 226868 Jan 27 at 18:17
@HDE226868 you're right it makes no sense. You should downvote when you see bogus / wrong / crazy answers :-) –  Brandon Enright Jan 27 at 21:03
@BrandonEnright Ah, right. Saw this in the queue and then had to go. Anyway, P.A.M: The Big Bang occurred roughly 9 billion years before the solar system formed, so it was unrelated to the planets. They began within the Hill Sphere (substituting that in for "gravitational field") of the Sun. They won't lose their velocity unless there's some extreme version of orbital decay at work. –  HDE 226868 Jan 27 at 21:22
@HDE226868 , brandonenright: what is the right answer then?! and i think u misunderstood my answer. –  P.A.M Mar 2 at 18:16

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