Here we see the bridge breaking away from its moorings. In this case, we had set any mass that we did not want to move to have a mass value of 0. This meant that it's location would not be modified in any way. To release it, all we did was set it to a non-zero value, and magic happens. The other thing we did was set the left top mass initial location to be in a different location. When this happens, AND the mass is 0, the mass will move -slowly, to this location if it is different from whereever it is currently.