# Planet Gravity Physics

This topic is 4958 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

## Recommended Posts

yahoo! I think I have done it! I have made a gravity Simulator for a Planetary Solar System! I have not implemented all 9 planets, Just the first 4. And the Hale Bopp comet. Take A look at this VB Source Code. You can download the .exe too. Can someome tell me if I have actually implemented the gravity correctly? I used 2D cartesian Vectors to combine the forces of gravity and planet momentum to get the new planet vectors. And why is it so hard to get a parabolic orbit, mostly they are circular or eliptical? Is it like this in reality? I guess so! Nightwalk

##### Share on other sites
A parabolic orbit?

If something is going very very fast you will get a hyperbolic and it will never come back. If something is going slowly enough it will form an ellipse. If it is going exactly at the point where it ceases to be an ellipse and becomes hyperbolic, it will be a parabola. The circular ones are just special cases of the elliptical ones.

##### Share on other sites
ohh i see, so it is just very unlikely to become a parabola?
So to become parabolic, Does the planet have to slightly drift out of orbit for a second and then come back in at a different ellipse?
That seems to make sense! cool
My simulator also doeesn't apply gravity of other planets to each other :(

NightWalk

##### Share on other sites
Why is venus going the wrong way? Don't all of the planets move in the same direction?

Keep it up though, let's see all the planets.

##### Share on other sites
Quote:
 Original post by Anonymous PosterWhy is venus going the wrong way? Don't all of the planets move in the same direction?

Nightwalk seems to have confused two things here: Venus is orbiting the sun in the same direction as the other planets, but Venus' rotation is retrograd.

You'll probably know that, but the planets and the sun are way too large compared to the distances between them (but else there would be barely visible dots....). The distances of the planets also don't seem to be correct - especially Jupiter should be farther from sun compared to the other planets.

If you want to make the simulator more realistic, besides calculating gravity of one planet acting on the other ones, you've to take into account that the planets are not in one plane and the sun is also moveable.

Quote:
 Original post by NightwalkSo to become parabolic, Does the planet have to slightly drift out of orbit for a second and then come back in at a different ellipse?That seems to make sense! cool

No, if the orbit is parabolic the planet will drift out of the system and won't come back.

##### Share on other sites
Quote:
 Original post by Nightwalkohh i see, so it is just very unlikely to become a parabola?So to become parabolic, Does the planet have to slightly drift out of orbit for a second and then come back in at a different ellipse?That seems to make sense! coolNightWalk

Your comet changes its orbit but that's probably only due to limited accuracy - if it is at its perihel the errors are larger because the acceleration is proportional to 1/r^2.

##### Share on other sites
Quote:
 Original post by NightwalkMy simulator also doeesn't apply gravity of other planets to each other :(NightWalk

Take a course in astronautics and you'll see that that's how most scientists model the universe as well. The two-body system makes modelling much easier and due to the fact that the sun is several orders of magnitude more massive than any other planet you can assume that its gravity force is also several orders of magnitude larger than the gravity force of any other planet on the other. Picture an object of mass 1e30 kg exerting a gravitic force at 1e8 m on a mass of 6e24 kg, F = Gc(Mm)/r^2 = Gc(1e30*6e24)/(1e8)^2 = Gc*6e38. Now consider a mass of 1e27 kg exerting a gravitic force at 7.5e8m on a mass of 1 6e24 kg, F = Gc(1e27*6e24)/(7.5e8)^2 = 1e34. That's the force of Jupiter on Earth. The force of Jupiter on earth is 4 orders of magnitude smaller than the force of the sun on earth, therefore it can basically be ignored in calculations.

##### Share on other sites
the shape of the orbit is determined by the energy of the system E =1/2*mv^2 - G*M*m/r
if this is <0 the orbit is circular or elliptical, if it is >0 the orbit is hyperbolic, if it is =0 the orbit is parabolic

##### Share on other sites
Thats a pretty cool demo.

##### Share on other sites
Quote:
Original post by jperalta
Quote:
 Original post by NightwalkMy simulator also doeesn't apply gravity of other planets to each other :(NightWalk

Take a course in astronautics and you'll see that that's how most scientists model the universe as well. ... The force of Jupiter on earth is 4 orders of magnitude smaller than the force of the sun on earth, therefore it can basically be ignored in calculations.

For planetary orbits, that's generally true. Mainly because planets stay within their orbits.

Things like comets, on the other hand, may pass very close to planets and be effected by them.

• 33
• 12
• 10
• 9
• 9
• ### Forum Statistics

• Total Topics
631352
• Total Posts
2999486
×