youBot is not attracted by gravity in Gazebo [closed]

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I am using the "youbot_gazebo_robot" package (Hydro) to simulate the youBot in Gazebo. This package uses the urdf of the youbot_description package. But something is wrong with the physics, the model does not seem affected by the gravity. For example, when the robot collides with an object, it happens it raises up on two wheels. Then it never goes down on the ground, it continues to move on two wheels (really cool indeed!!). How can I fix this problem?