Version: gazebo7, indigo, ubuntu14

Background: Robot arm model and box model are added to the world respectively in separate launch file. At first, box stays well in front of the robot arm.

Problem: static box keeps incline after is collided with moving robot arm, why the box doesn't fall down on the ground?

Here is my box.xacro script, the inertia tag is well defined according to formula of moments of inertia of uniform box

If you need more info, please leave a comment. Thanks!