Game object disappears after a dozen seconds.
Well, OK, now I know why but I'll ask the question and answer it anyway; maybe it'll help someone save some time.
Process: Start with the default startup file. Switch to game engine. Switch to Game Logic. Select the cube. On physics menu pick any of: dynamic, rigid body, soft body, or character. Press play. Cube object disappears after a dozen seconds. Can't figure why. My searches tuned up nothing relevant. I've tried using Blender 2.77 64bit and 2.76b 32bit. Same results in both. I'm running WINDOWS 10.
Answer: The default game engine 3D view places the cube in top view, looking down the z-axis. Because it's an orthographic view there is no perspective and you cannot tell that the cube is FALLING ( down the z-axis ). After about 10-12 seconds it reaches the clipping distance and vanishes. DOH!