How to make select parts of composite models static

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Currently, it isn't possible to make some links of a model static while the others are dynamic.

A workaround would be to fix the platform to the world with a joint that has very low limits to make it almost fixed (you can try making it completely fixed by setting upper and lower limits == 0, but that might cause instability). I'd also make it very heavy to make sure it doesn't move. See the foosball table as an example, it would look something like this:

<joint name='platform_world' type='prismatic'>
  <parent>world</parent>
  <child>platform</child>
  <axis>
    <xyz>0 0 1</xyz>
    <limit>
      <lower>0</lower>
      <upper>0.1</upper>
    </limit>
  </axis>
</joint>