Simple robot bounces when placed in world

I have a simple structure that is a vertical pole connected to a horizontal pole at its top (forming an upside-down-L shape). A box hangs from the end of the horizontal pole. Since the structure isn't balanced, I added a joint between the body and the world to keep it from falling down. It doesn't fall down anymore, but now when I place it in the world it keeps bouncing up and down a small amount. Please let me know if you have any advice! Here's my model.sdf code:

<link name='body'>
<pose>0 0 0 0 0 0</pose>
<collision name='collision'>
<geometry>
<box>
<size>.1 .1 3</size>
</box>
</geometry>
</collision>

<visual name='visual'>
<geometry>
<box>
<size>.1 .1 3</size>
</box>
</geometry>
</visual>

<collision name='collision'>
<pose>2.5 0 1.5 0 0 0</pose>
<geometry>
<box>
<size>5 .1 .1</size>
</box>
</geometry>
</collision>

<visual name='visual'>
<pose>2.5 0 1.5 0 0 0</pose>
<geometry>
<box>
<size>5 .1 .1</size>
</box>
</geometry>
</visual>

<pose>5 0 2 0 0 0</pose>

<collision name='collision'>
<geometry>
<box>
<size>1 1 1</size>
</box>
</geometry>
</collision>

<visual name='visual'>
<geometry>
<box>
<size>1 1 1</size>
</box>
</geometry>
</visual>

<joint type="revolute" name="box_hinge">
<pose>0 0 0 0 0 0</pose>
<child>heavy_box</child>
<parent>body</parent>
<axis>
<xyz>0 0 1</xyz>
<limit>
<lower>-0.001</lower>
<upper>0.001</upper>
</limit>
</axis>
</joint>

<joint type="revolute" name="ground_hinge1">
<pose>0 0 0 0 0 0</pose>
<child>body</child>
<parent>world</parent>
<axis>
<xyz>0 0 1</xyz>
<limit>
<lower>-0.001</lower>
<upper>0.001</upper>
</limit>
</axis>
</joint>

</model>
</sdf>

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