[ODE] Adding Negative Masses

[Jaroslav Sinecky]

I think this is to much of an adventure to let ode simulate body with 
negative mass. It's not just about gravity, it's about inertia. What 
should equations of motion do when they encounter negative inertia? 
Accelerate the object when some force try to stop it? (as when hitting 
the plane)
For a balloon I would prefer to just apply upward force at each step :-)

For dMassAdd, the idea of negative mass is not bad, but I don't dare to 
say if it will work. Anyway I see assert for mass being positive in 
checkMass function. Maybe dMassSub would be better.

Jaroslav


LR wrote:
> I've just tested and ... The geom with negative mass (encapsulated in a
> transformgeom) is falling and not collide with my plane ...
>
> Laurent.
>
> -----Message d'origine-----
> De : ode-bounces at q12.org [mailto:ode-bounces at q12.org] De la part de Bram
> Stolk
> Envoyé : mercredi 26 avril 2006 10:01
> À : Wesley Smith
> Cc : ODE Mailing List (E-mail)
> Objet : Re: [ODE] Adding Negative Masses
>
> Wesley Smith wrote:
>   
>> Hi,
>> I'm curious is if it's possible to use dMassAdd() to add a negative 
>> mass to a large mass and have the result be as if a chunk of that mass 
>> went missing of the corresponding form.  For  example, if I do 
>> dMassAdd(), combining 3 objects, and now want to take one of them 
>> away.  Assuming that I've kept some record of the original small 
>> body's mass, can I negate it's mass params in the dMass struct and 
>> then call dMassAdd(&compositeMass, &body2) and have it behave as if 
>> body 2 had been taken away?  I'm guessing yes, but I want to be sure.
>>
>>     
>
> Hey... nice idea.
> I can't say for sure, but my guess is that it works.
> You'll just have to try.
>
> It would be nice to have an example that simulates a helium balloon in ODE.
> Or a hindenburg? Cool! I just may try that in my spare time :-)
>
>  Bram
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