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Code archives/Algorithms/2d planetary motion

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2d planetary motion by Nate the Great2009
This is a little 2d planet motion simulator I made in bmax. The damper variable is the key to slowing down or speeding up gravity and how it works. I came up with this algorithm on my own so it is not the most efficient but it works. 
SuperStrict

Graphics 1440,900,0,60

Global plist:TList = New TList
Global damper:Float = .0001
Global timer:ttimer = CreateTimer(60)
Global mxspdx:Float = 0
Global mxspdy:Float = 0
Global omx:Int = MouseX()
Global omy:Int = MouseY()
planet.Create(1440/2,900/2,0,0,10000,50,255,255,0,1)
While Not KeyDown(key_escape)
Cls

mxspdx = MouseX()-omx
mxspdy = MouseY()-omy
omx = MouseX()
omy = MouseY()
DrawText mxspdx+"  "+mxspdy,1,1

updateplanets()

If MouseHit(1) Then
	Local mas# = Rnd(10,20)
	planet.Create(MouseX(),MouseY(),mxspdx / 30,mxspdy / 30,mas,mas*1.1,Rnd(100,255),Rnd(100,255),Rnd(100,255),.9)
EndIf

If MouseHit(2) Then
	Local mas# = Rnd(10,200)
	mas = mas / 200
	planet.Create(MouseX(),MouseY(),mxspdx / 30,mxspdy / 30,mas,mas*1.1,Rnd(100,255),Rnd(100,255),Rnd(100,255),.9)
EndIf
WaitTimer(timer)
Flip False
Wend
End

Type planet
	Field x#
	Field y#
	Field dx#	'velocity x
	Field dy#	'velocity y
	Field mass#
	Field rad#	'radius
	Field r:Int
	Field g:Int
	Field b:Int
	Field alph#	'alpha
	Field don:Int
	
	Function Create:planet(x#,y#,dx#,dy#,mass#,rad#,r:Int,g:Int,b:Int,alph#)
		Local p:planet = New planet
		p.x = x
		p.y = y
		p.dx = dx
		p.dy = dy
		p.mass = mass
		p.rad = rad
		p.r = r
		p.g = g
		p.b = b
		p.alph = alph
		
		plist.addlast(p:planet)
	End Function
	
	Method draw()
		SetColor r,g,b
		SetAlpha alph#
	
		If rad > 1 Then
			DrawOval x-rad,y-rad,2*rad,2*rad
		Else
			Plot x,y
		EndIf
	End Method
	
	Method update()
		x = x + dx
		y = y + dy
	End Method
	
	Method dist:Float(p:planet)	'returns distance squared
		If p <> Self Then
			Return (p.y - y)^2 + (p.x - x)^2
		EndIf
	End Method
End Type


Function updateplanets()

For Local p:planet = EachIn plist
	p.draw
	p.update
	p.don = True
	For Local p2:planet = EachIn plist
		If Not p2.don Then
			Local d# = p.dist(p2:planet)	' a shortcut for using 1/d^2: just dont use sqr in the distance formula and it turns into 1/d
			If d > 0 Then
				Local dx# = p.x - p2.x
				Local dy# = p.y - p2.y
				p.dx = p.dx - (dx/(d))*p2.mass*damper
				p.dy = p.dy - (dy/(d))*p2.mass*damper
				p2.dx = p2.dx + (dx/(d))*p.mass*damper
				p2.dy = p2.dy + (dy/(d))*p.mass*damper
			EndIf
		EndIf
	Next
Next

For Local p:planet = EachIn plist
	p.don = False
Next
End Function

Comments

Krischan2009
Oh, here is my own version, very old but funny.

Keyboard commands
Arrows up/down = Zoom in/out
Arrows left/right = slower/faster
Space = Tracers on/off
F1 = inner System
F2 = complete solar system
F3 = double sun mass
F4 = half sun mass
F5 = Earth get 0.05 sun masses




Nate the Great2009
haha thats pretty cool!
so if the sun's mass is doubled and then when the earth is going fastest, it is halfed, the earth is shot away and never comes back!


Krischan2009
The problem is that the precision gets lost if you increase the time too much. In reality, a 15-meter measurement inaccuracy of earth's orbit will lead to a 100% prediction loss of earth's orbit in 100 Million years! And Blitz only has 6 decimal places precision.

A step of 1 is one second per day (approx.), a step of 1/3600 = 0.0002777 is more real but very boring to watch :-)

Oh if somebody is interested in these mathematics, here is a nice website with many (Quick-)Basic sources:

http://www.physics.odu.edu/~gcopelan/cai-lib/simulations.htm


slenkar2009
the planets go flying off when they reach the sun


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