# Where do planets get energy to revolve around sun? [duplicate]

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?

• Possible duplicates: physics.stackexchange.com/q/5905/2451 and physics.stackexchange.com/q/9049/2451 Commented Mar 8, 2013 at 15:24
• 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. Commented Mar 8, 2013 at 15:25
• Commented Mar 8, 2013 at 17:10
• they don't need energy -- it's a closed path, so the work done is 0. Commented Jun 2, 2018 at 4:59
• Actually i wanted to know were did it get the energy to overcome the gravitational force, not the energy to move in the path, which surely is 0. Commented Jun 2, 2018 at 16:29

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.

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 intents and 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.