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Why do some models, or objects in a MuJoCo XML script fall to the floor while others don't?

In the models files that come with the MuJoCo zip, the robot arm (arm26) does not fall to the ground while the humanoid and many other models have objects that fall to the ground.
I think I've compared every facet of the files and haven't been able to pinpoint what causes this in some files but not for others.

I will include the code of the arm26 file, which does not fall to the ground.
An answer to this question would be a modification to this code that would make it fall to the ground.

<mujoco model="2-link 6-muscle arm">

    <option timestep="0.005" iterations="50" solver="Newton" tolerance="1e-10"/>

    <size njmax="50" nconmax="10" nstack="200"/>

    <visual>
        <rgba haze=".3 .3 .3 1"/>
    </visual>

    <default>
        <joint type="hinge" pos="0 0 0" axis="0 0 1" limited="true" range="0 120" damping="0.1"/>
        <muscle ctrllimited="true" ctrlrange="0 1"/>
    </default>

    <asset>
        <texture type="skybox" builtin="gradient" rgb1="0.6 0.6 0.6" rgb2="0 0 0" width="512" height="512"/>

        <texture name="texplane" type="2d" builtin="checker" rgb1=".25 .25 .25" rgb2=".3 .3 .3" width="512" height="512" mark="cross" markrgb=".8 .8 .8"/>

        <material name="matplane" reflectance="0.3" texture="texplane" texrepeat="1 1" texuniform="true"/>
    </asset>

    <worldbody>
        <geom name="floor" pos="0 0 -0.5" size="0 0 1" type="plane" material="matplane"/>

        <light directional="true" diffuse=".8 .8 .8" specular=".2 .2 .2" pos="0 0 5" dir="0 0 -1"/>

        <site name="s0" pos="-0.15 0 0" size="0.02"/>
        <site name="x0" pos="0 -0.15 0" size="0.02" rgba="0 .7 0 1" group="1"/>

        <body pos="0 0 0">
            <geom name="upper arm" type="capsule" size="0.045" fromto="0 0 0  0.5 0 0" rgba=".5 .1 .1 1"/>
            <joint name="shoulder"/>
            <geom name="shoulder" type="cylinder" pos="0 0 0" size=".1 .05" rgba=".5 .1 .8 .5" mass="0" group="1"/>

            <site name="s1" pos="0.15 0.06 0" size="0.02"/>
            <site name="s2" pos="0.15 -0.06 0" size="0.02"/>
            <site name="s3" pos="0.4 0.06 0" size="0.02"/>
            <site name="s4" pos="0.4 -0.06 0" size="0.02"/>
            <site name="s5" pos="0.25 0.1 0" size="0.02"/>
            <site name="s6" pos="0.25 -0.1 0" size="0.02"/>
            <site name="x1" pos="0.5 -0.15 0" size="0.02" rgba="0 .7 0 1" group="1"/>

            <body pos="0.5 0 0">
                <geom name="forearm" type="capsule" size="0.035" fromto="0 0 0  0.5 0 0" rgba=".5 .1 .1 1"/>
                <joint name="elbow"/>
                <geom name="elbow" type="cylinder" pos="0 0 0" size=".08 .05" rgba=".5 .1 .8 .5" mass="0" group="1"/>

                <site name="s7" pos="0.11 0.05 0" size="0.02"/>
                <site name="s8" pos="0.11 -0.05 0" size="0.02"/>
            </body>
        </body>
    </worldbody>

    <tendon>
        <spatial name="SF" width="0.01">
            <site site="s0"/>
            <geom geom="shoulder"/>
            <site site="s1"/>
        </spatial>

        <spatial name="SE" width="0.01">
            <site site="s0"/>
            <geom geom="shoulder" sidesite="x0"/>
            <site site="s2"/>
        </spatial>

        <spatial name="EF" width="0.01">
            <site site="s3"/>
            <geom geom="elbow"/>
            <site site="s7"/>
        </spatial>

        <spatial name="EE" width="0.01">
            <site site="s4"/>
            <geom geom="elbow" sidesite="x1"/>
            <site site="s8"/>
        </spatial>

        <spatial name="BF" width="0.009" rgba=".4 .6 .4 1">
            <site site="s0"/>
            <geom geom="shoulder"/>
            <site site="s5"/>
            <geom geom="elbow"/>
            <site site="s7"/>
        </spatial>

        <spatial name="BE" width="0.009" rgba=".4 .6 .4 1">
            <site site="s0"/>
            <geom geom="shoulder" sidesite="x0"/>
            <site site="s6"/>
            <geom geom="elbow" sidesite="x1"/>
            <site site="s8"/>
        </spatial>
    </tendon>

    <actuator>
        <muscle name="SF" tendon="SF"/>
        <muscle name="SE" tendon="SE"/>
        <muscle name="EF" tendon="EF"/>
        <muscle name="EE" tendon="EE"/>
        <muscle name="BF" tendon="BF"/>
        <muscle name="BE" tendon="BE"/>
    </actuator>
</mujoco>
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  • $\begingroup$ please add an XML file for an object that falls to floor $\endgroup$
    – jsotola
    Nov 14, 2021 at 1:21
  • $\begingroup$ I pinpointed the line that makes the bodies fall. <freejoint name="root"/>. Though I've gotten it to work with a simple block, I just tried for 20 minutes to integrate it into the code above and kept getting errors. Such as 'error: mass and intertia of moving bodies must be positive`. $\endgroup$
    – Ant
    Nov 14, 2021 at 15:02
  • $\begingroup$ Are you still needing help with this or do you have it figured out? $\endgroup$
    – Chuck ♦
    Nov 15, 2021 at 14:50
  • $\begingroup$ I think the answer might be something that brings in the geom contype. I don't fully understand it but I'm learning. So, unless you wanted to post an answer about what's going on with that I'll just grind at it myself. I'm getting there, thank you. $\endgroup$
    – Ant
    Nov 15, 2021 at 17:38

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