Now I have two models and one of them is a moving robot, the other is a football. I set the velocity of the robot and the ball to be the same. Then I plot the differences between their velocities and their distance. The image below is captured from rqt_plot; data[0](blue line) is the distance between robot and the football and data[1](red line) is the difference between their velocities.

As you can see, there is almost no differences betwwen their velocities, but the distance is vibrating up and down all the time and it never stops. What's this wired situation?

I use Gazebo 5.1 and ODE physics engine. ROS Indigo and gazebo_ros_pkgs is installed in Ubuntu 14.04 Trusty 64 bits.