# [ODE] Adjusting a body's mass

Geoff Carlton gcarlton at iinet.net.au
Tue Mar 15 11:56:04 MST 2005

```Yes, it sounds perfect.  Unfortunately, its not.  What I mentioned is
for the distribution around the centre, but the actual centre position
needs to be set according to what Jon has mentioned.

Geoff

Johnson, Erik USA wrote:

>Sounds perfect!
>
>Let's see, does that mean I have to use the dMassSetParameters() method?
>Aside from dMassTranslate(), this is the only method with access to the
>center of gravity position.
>
>Thanks
>
>
>
>
>>-----Original Message-----
>>From: ode-bounces at q12.org [mailto:ode-bounces at q12.org] On
>>Behalf Of Geoff Carlton
>>Sent: Monday, March 14, 2005 4:15 PM
>>To: ode at q12.org
>>Subject: Re: [ODE] Adjusting a body's mass
>>
>>In addition, the distribution of weight is determined by the
>>dMass struct, since the geom shape has no affect on the
>>actual physics beyond determining the contacts.  The dMass
>>determines the inertia of the body, which can be a fairly
>>subtle effect - it will change the way the body spins in the
>>air, how it tumbles, etc.  So even if the centre of gravity
>>doesn't change, dMass will be needed to get proper results -
>>you could have all the weight in the middle, or if you have a
>>massive pair of weights at the front and the back.
>>
>>Geoff
>>
>>Jon Watte wrote:
>>
>>
>>
>>>Mass translation is ignored by the body, so you have to
>>>
>>>
>>caculate what
>>
>>
>>>the actual center of gravity is, and put the body there
>>>
>>>
>>(typically by
>>
>>
>>>offsetting any geoms and joint attachment points from the body).
>>>
>>>A second, inferior, solution is to add a number of bodies to
>>>
>>>
>>the main
>>
>>
>>>body, all attached with fixed joints. This will perform
>>>
>>>
>>
>>
>>>to less stability in the simulation.
>>>
>>>Cheers,
>>>
>>>			/ h+
>>>
>>>
>>>
>>>
>>>
>>>>In my particular case, I want to add and remove "weight" in certain
>>>>places on my vehicle chassis such as a fuel tank, or a driver, or a
>>>>rocket launcher.  My hope is that I can make the vehicle
>>>>
>>>>
>>front or back "heavy" and it will affect the steering.
>>
>>
>>>>
>>>>
>>>>
>>>>
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>>>
>>>
>>>
>>>
>>>
>>>
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>
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```