[ODE] air friction

[Ivan Bolcina]

I see. But don't you have a feeling that this is something that ODE 
should be rensonsibe to do. Doing it from outside, it feels that
am performing some sort of unnatural thing on otherwise closed world. 
Suddenly one object just stops beheaving under physical laws, and it stops.
Miracle! Wouldn't it be more natural just to operate with forces and 
torques, and maybe initial positions. SetVelocity and SetPosions would 
better be used just
for inital setup of world.

BTW, I my simulation I get to stop object just by appliying forces. But 
I have to check the time step and adjust force to it. And I have a 
feeling it stops just because onced the
velocity is so small, that speed vector becomes 0.

bye



DjArcas wrote:

>Also, except in test cases, your world is rarely an infinite plane. ;)
>
>----- Original Message ----- 
>From: "Ivan Bolcina" <ivan.bolcina at snt.si>
>To: <ode at q12.org>
>Sent: Wednesday, April 21, 2004 3:25 PM
>Subject: Re: [ODE] air friction
>
>
>  
>
>>Actually I have a question on this to.
>>If you want to simulate drag of friction, than you put some force. The
>>problem is, the force will work fore some ammount of time, giving the
>>object certain  amount of dV.
>>The problem is,that by doing this, the object never stops (or it stops,
>>but only because float numbers have limited range).  This is also true
>>for air friction, I think.
>>The object in realitiy would never completly stop, since friction force
>>would alwasy produce smaller dV for current velocity. But this would
>>only be true, if we could operate with smaller and smaller dt.  If we
>>operate with fixed dt, I am not sure the results are accurate.
>>
>>The sistem doesn't seem stable. During some point, the ODE should decide
>>that object is moving so slowly, that it is actually stoped.
>>BTW, friction and drag are actually sum of forces of molecules hitting
>>the object, or some other micro events, and we have to simulate this in
>>one way or another. The question is how?
>>
>>
>>DjArcas wrote:
>>
>>    
>>
>>>Uhm, you could read the FAQ.
>>>
>>>Q How do I stop things from rolling off into infinity, or pendulums from
>>>swinging forever?
>>>A ODE models a universe with frictionless hinges and no rolling friction,
>>>      
>>>
>so
>  
>
>>>problems like those are fairly common. The solution is to apply a force
>>>      
>>>
>to
>  
>
>>>simulate RollingFrictionOrAerodynamicDrag.
>>>
>>>http://q12.org/cgi-bin/wiki.pl?RollingFrictionOrAerodynamicDrag
>>>
>>>
>>>----- Original Message ----- 
>>>From: <barbara.yersin at urbanet.ch>
>>>To: <ode at q12.org>
>>>Sent: Wednesday, April 21, 2004 10:58 AM
>>>Subject: [ODE] air friction
>>>
>>>
>>>
>>>
>>>      
>>>
>>>>Hi guys!
>>>>I now have a ball  on which I apply a force once (only once => no
>>>>
>>>>
>>>>        
>>>>
>>>acceleration, constant speed)
>>>
>>>
>>>      
>>>
>>>>Then the ball starts rolling, but it never stops. How can I create the
>>>>        
>>>>
>air
>  
>
>>>>        
>>>>
>>>friction effect? How can I stop the ball ?
>>>
>>>
>>>      
>>>
>>>>Thanks!
>>>>
>>>>Barbara
>>>>
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>>>>
>>>>        
>>>>
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>>>      
>>>
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>>
>
>  
>