DR.Orbit

Demonstration of how a mass (e.g. earth) can orbit around a gravitational well (e.g. sun). Use your finger to push the mass onto a trajectory that results in a stable orbit around the well. Use the options menu to introduce some drag (good to get rid of the mass), to bring up some eye candy or to influence the movement of the mass by tilting your iPad.

Physics background:

Gravitational forces act between masses. In this demonstration a big stationary mass is situated in the center of the screen. Its location is fixed and it is hidden behind a cap. If you put a small mass onto the screen, it begins accelerating towards the big mass until it disappears under the cap "into the drain". You can prevent your mass from going down the drain by giving it an initial push. Try experimenting with the directions of your pushes. If it is done right, the small mass will be falling around the big mass without - in theory - ever disappearing into the drain. But due to the not very accurate numerical methods used for the calculation of the trajectory of the mass, the reality in this demonstration might be different. This also applies to the background grid on the screen: It does not show the correct wraping of space caused by the big mass in the middle of the screen. Its shape is a compromise between aesthetics and numerics. 

Link to demonstration video:
http://www.youtube.com/watch?v=pwXI-EiZE4o