Orbits, Gravity, and You
The gravitational force between two bodies is G m1 m2 / (r^2) where G is a constant (6.67 x 10^-11), m1 is mass of body 1, m2 is mass of body 2, and r is the distance between the centers of mass of the two bodies.
quote:Original post by cowsarenotevil
Wouldn''t "air" resistance eventually cause planets to fall into the sun, extremely slowly?
there is no air in space...
lol 
a body move in a straight line motion until a force acts on it and changes his direction. Something like that... anyway, if the gravitiational pull stopped acting, the planet in orbit would fly in to space. The gravity pulls the planet inwards, but the planet''s momentum keeps it in orbit.
a simple gravitational force, as explained by space dude, with a good integrator (runge-kutta) will give you good results. You neeed good starting conditions, like the position of the planet and their velocity, so they stay in orbit. Alternatively, you can just populate a world with, say 10,000 particles, with random velocitites, and let the system stabilise itself. Eventually, it will turn into galaxies. Very cool. Obviously, with that many attractors, you need to precompute it, and play it back at 30 fps.
a body move in a straight line motion until a force acts on it and changes his direction. Something like that... anyway, if the gravitiational pull stopped acting, the planet in orbit would fly in to space. The gravity pulls the planet inwards, but the planet''s momentum keeps it in orbit.
a simple gravitational force, as explained by space dude, with a good integrator (runge-kutta) will give you good results. You neeed good starting conditions, like the position of the planet and their velocity, so they stay in orbit. Alternatively, you can just populate a world with, say 10,000 particles, with random velocitites, and let the system stabilise itself. Eventually, it will turn into galaxies. Very cool. Obviously, with that many attractors, you need to precompute it, and play it back at 30 fps.
if solar mass loss, planetary mass loss/win and solar radiation pressure (and other non-gravitational forces) were neglectable, planets would spiral in very slowly thereby radiating gravitational waves, according to relativity.
The reduction of orbital periods of tightly orbiting neutron stars due to emission of gravitational waves was actually measured and the measurements confirmed general relativity.
But i think general relativity iss hardly relevant for your simulation
ga
[edited by - ga on November 9, 2003 10:13:02 AM]
The reduction of orbital periods of tightly orbiting neutron stars due to emission of gravitational waves was actually measured and the measurements confirmed general relativity.
But i think general relativity iss hardly relevant for your simulation
ga
[edited by - ga on November 9, 2003 10:13:02 AM]
quote:Original post by SpaceDude
there is no air in space...
Actually there's lots of rarified gas floating around up there. Not enough to breathe certainly, but, unless you're in deep, deep space, you're certainly not in a vacuum. So sure there's resistance. It's very small, but it's there.
This is a seperate issue from gravity waves, which radiate gravitational energy, but otherwise of which I know next to nothing.
There are also formulae for the actual ellipses in which planets travel. The sun is at one focus. You could store these parameterizations of the motion and avoid integration altogether.
[edited by - TerranFury on November 9, 2003 2:48:43 PM]
quote:
Actually there''s lots of rarified gas floating around up there. Not enough to breathe certainly, but, unless you''re in deep, deep space, you''re certainly not in a vacuum. So sure there''s resistance. It''s very small, but it''s there.
however much ''air'' there is in space, it orbits along with the planets anyway, otherwise it would have fallen into the sun long since
.
There is no vaccum in space. Quantum Mechanics explains this. Space is filled with lots of particles and energy that are very small and most last for only minute times as they go back to spacetime.
Well, our solar system (as many body system) is not stable at all, so why bother with friction.
quote:Original post by TerranFury
There are also formulae for the actual ellipses in which planets travel. The sun is at one focus. You could store these parameterizations of the motion and avoid integration altogether.
That kinda takes away from the fun of trying to simulate the solar system... I mean, if you programmed it all from basic physics principles you could have some fun and see what happens when you add an extra planet to the solar system... etc...
quote:Original post by Eelcoquote:
Actually there''s lots of rarified gas floating around up there. Not enough to breathe certainly, but, unless you''re in deep, deep space, you''re certainly not in a vacuum. So sure there''s resistance. It''s very small, but it''s there.
however much ''air'' there is in space, it orbits along with the planets anyway, otherwise it would have fallen into the sun long since
it probably would have been blown away by solar wind and radiation pressure. fine dust grains (smaller than about 0.1 µm, depending on optical properties) are also blown away by radiation pressure, slightly larger particles are orbiting the sun in slower orbits than the planets because radiation pressure partially compensates the gravitational force (radiation pressure also decreases with the square of the distance from sun)
ga
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