Game object disappears after a dozen seconds. 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!
Ensure to have a ground within the cameras viewing frustum. Or better look from the side to see the cube falling