Understanding Orbital Mechanics: The Force Behind Planetary Motion

[Kothar]

Hello I was searching a community like this one and I am glad I found one.

I have a question about the beginning of the universe. I am trying to make a simulation of big bang, as I know after the big bang enormous amount of matter scatter around and they started to combine and turn into bigger masses. Ok, here is the point;

After the stars (bigger masses with mostly Hydrogen) and the planets (smaller masses) are formed they started systems. But how, how can these huge masses started to revolve around, what force caused them to not to collide with each other like the smaller masses at the beginning but instead make them to revolve around each other and around themselves.

People say use a string a stick and a mass and connect the mass to stick using the string when you turn it the tension on the string represents gravity. But still it was the kinetic force we applied on the stick and somehow a force made the planets revolve the same way. I am asking what is that force?

Sorry for the bad English + long question and thank you for your answers.

 

[DaveC426913]

...what force caused them to not to collide with each other...

They did. That's how we got larger masses.

You're thinking in too broad steps. For one, stars and planets did not form "from" the Big Bang, or even shortly after.

What followed the BB was a universe filled with hot dust and gas. Stars formed when these clouds of dust and gas began collapsing under gravity. As they collapsed, they preserved the angular momentum of the cloud from which they formed. There was much crashing about, but not all of it was collisions. Anything that did not collide ended up either missing each other altogether or in stable orbits.

 

[Kothar]

Thank you for the answer. I did some research and found about Protoplanetary disk which supports your answer. But I am wandering how these dust clouds got the angular momentum.

The formed masses which later turned into stars and planets, why they are turning around themselves.

Thank you

 

[DaveC426913]

Thank you for the answer. I did some research and found about Protoplanetary disk which supports your answer. But I am wandering how these dust clouds got the angular momentum.

The formed masses which later turned into stars and planets, why they are turning around themselves.

Thank you

Pretend there are two specks of dust in an otherwise empty volume of space.
Give these two specks of dust some random motion relative to each other (a random speed in a random direction. Note that it would be a fabulous coincidence if the two particles had zero tangential velocity wrt each other. Unless you gave them motions that are exactly toward each other or exactly away from each other, they'll have some tangential motion.)

Now consider the centre of mass of this "system". You can see that, if you consider the entire system, the specks are already rotating with respect to each other.


Now allow gravity to act upon them, drawing them together. They will fall toward each other. One of three things will happen:
1] They will slingshot around each other, having too much kinetic energy to capture each other, and will go shooting off into the unknown, never to interact again.
2] They will collide. But remember, these are specks in a volume of space. In order to collide they would have had to have zero tangential motion. Very unlikely.
3] They shoot past each other, but have small enough kinetic energy differential that they are caught by each other's gravity and fall back toward each other. They are in orbit.

The coalescence of bodies and their orbital motions is an inevitable consequence of nothing more than their gravitational attraction and their initial random motion.

 

[texasblitzem]

Dave, your response was the most concise and clearest explanation I've ever read regarding this topic. I've wondered the same thing as Kothar and I'm glad he asked it. Thanks Dave, I can totally visualize your specks' interactions with each other, I get it now.

 

[Kothar]

Thank you for your answers Dave, it starts to make sense, I can figure it out the rest on my own I guess. When I finish the simulation I'll be glad to share it in here.

 

[DaveC426913]

Thank you for your answers Dave, it starts to make sense, I can figure it out the rest on my own I guess. When I finish the simulation I'll be glad to share it in here.

Instead of making a simulation based on your current knowledge, wouldn't that time be better spent learning about BB theories and simulations already done by cosmologists?

 

[Kothar]

Well... this simulation is for a course project (We must do a graphic based programs with physics engines we will write), also by making my own I can learn more on the way. Also are there any chance that I can look these simulations you mentioned?

 

[DaveC426913]

Well... this simulation is for a course project (We must do a graphic based programs with physics engines we will write), also by making my own I can learn more on the way. Also are there any chance that I can look these simulations you mentioned?

Well, I didn't mean (or know of) anything specific, I just meant finding and reading some papers on the subject.


BB is one simulation, but you know, another might simply be star sytems. It's pretty straightforward to write a gravity simulator, drop a few stars/planets into it and watch them pirouette round each other.

 

[Kothar]

Basically my first step will be a gravity simulator like this one http://www.nowykurier.com/toys/gravity/gravity.html then piece by piece I ll try to add what I've learned during the process. And I guess it will be fun seeing there are lots of BB theories.

What I have in mind is not just big bang, but the formation of all the galaxies as we know today. BB -> dust -> Huge Stars -> Super Nova -> Protoplanetary disks -> Systems and Galaxies. That is the process I want to simulate. Though there will be less particles than in a real galaxy :)

 

[DaveC426913]

Basically my first step will be a gravity simulator like this one http://www.nowykurier.com/toys/gravity/gravity.html then piece by piece I ll try to add what

That's a sweet simulator. I've built a couple like that but never thought to deal with collisions. Mine just continued to orbit.

I love playing with these simulators to see if I can get a stable 3-body system going. (i.e. a Sun-Earth-Moon system) It's virtually impossible.

 

[ThomasEdison]

I don't particularly want to start a separate thread since this question closely ties into the period of time after BB that those many conglomerations of gases formed.

The part that puzzles me is: what was stopping a homogenous distribution of mass all throughout the early Universe?

If it were homogenous then wouldn't gravity have been moot since all parts would be pulling together equally from all directions. Wouldn't that process pull the Universe back into a singularity once again (if all points were the same)?

So why did some portions of this cloud have more density than others to start this process?

What caused the fractures, cracks, imbalance? (whatever term is mentioned?)

 

[DaveC426913]

So why did some portions of this cloud have more density than others to start this process?

What caused the fractures, cracks, imbalance? (whatever term is mentioned?)

That is an area of active research. BB models suggest that the initial space should have been perfectly uniform, which leads to a homogenous universe.


But note that, in order for our universe to look the way it does, the initial imbalances can have been exceedingly small. The process is self-reinforcing. The tiniest imbalances will multiply rapidly until the unvierse becomes stringy and knotty.

The inflation theory attributes the origin of large scale structures to quantum fluctuation (of the scalar field), which occurred near the beginning of Big Bang. The fluctuation is subsequently enlarged by the inflation and served as a blue-print for the large scale structures such as the superclusters.

http://universe-review.ca/F03-supercluster.htm

 

[texasblitzem]

I love playing with these simulators to see if I can get a stable 3-body system going. (i.e. a Sun-Earth-Moon system) It's virtually impossible.

Dave, I've been playing with the simulator in the link a few posts up. What constitutes as a stable 3-body system? I was able to get a large mass orbited by a tiny mass and those two orbited an OMFG mass. But the orbital path was a bit eccentric, and the "sun" very slowly traveled across the screen. It lasted until I got tired of watching it. Was that a stable system?

I've been playing with this simulator for hours at work everyday, it's a blast. Is there anything else to attempt with this simulator that's a challenge? One challenge I'm having is getting two identical masses orbiting each other in a circular orbit. Do you think that's possible?

 

[DaveC426913]

Dave, I've been playing with the simulator in the link a few posts up. What constitutes as a stable 3-body system? I was able to get a large mass orbited by a tiny mass and those two orbited an OMFG mass. But the orbital path was a bit eccentric, and the "sun" very slowly traveled across the screen. It lasted until I got tired of watching it. Was that a stable system?

Yeah. Way to go. I had a lot of trouble with that.

As for the slow migraton across the screen, I found a way to deal with that. You can impart momentum on the central mass by "feeding" it the momentum of smaller masses.

If M is drifting to the left, along with its companion M₁ and moon M₁₁ , I'll create a small mass M_n just to the right of M, and give it a hefty rightward velocity. The velocity is not enough to have M_n escape, so it immediately gets absorbed by M. But M_n's momentum is not lost; it is added to M's momentum, which will serve to stop the migration.

The downside of course, is that you are adding mass to M, which will affect the orbit of M₁.

Then again, even that can be put to use. If you add multiple M_ns at the right time in M₁'s orbit (at perihelion), it will help to circularize M₁'s orbit.

Now you know how God felt 6000 years ago!

 

[Tanelorn]

I managed this without help. Do I get a chocolate biscuit? :)

Actually the best way is to hold down the CTRL key and then move the screen around by pressing the LH mouse button!