ball falling through ground instead of laying on ground plane

This should be easy, but I'm missing something obvious. I'm trying to create a urdf file for a ball that that I can launch multiple times at different locations through roslaunch. My attempt at urdf is :  I first tried without the gazebo tag, but in either case the ball gets added to the gazebo simulation, but then it is not visible . I can select the model "ball_1" in Gazebo, but the pose continues to go negative as if it fell through the ground plane. The robot model is working as expected. I'm running ROS jade under Ubuntu 14.04 with a separate gazebo5 install