The following simulation show gravitation field as function of r.
You can change the radius of the planet with the slider.
You can also drag mouse to any position and the simulation will show gravitation field at the new mouse position.
I hope this is what you need!

translate strings in simulation to different language format before download
Full screen applet or Problem viewing java?Add to exception site list
Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop. This work is licensed under a Creative Commons Attribution 2.5 Taiwan License
Download EJS jar file(1100kB):double click downloaded file to run it. (56 times by 19 users) , Download EJS source View EJS source