Two objects, red ball and blue ball, are connected with a spring. The Black dot shows the position of the center of the gravity of the two balls.
In the lower window, the camera moves along with the center of the gravity, which shows the retative motion of the two ballsz to the center of the gravity.
Controls:
Upper window: double click to change the position of the camera.
Lower window: single click or a tap to look at the red or blue ball from the center of the gravity.