TP's Force operator can be used to simulate gravity.
When using units where 1 unit = 0.1 Meter, and calculating distance over time with:
d = (9.8066 * 10.0) * ( (frames / 24.0)^2 ) / 2
Note: (mult 9.8066 by 10.0 to convert units to meters, and frames/24.0 to get distance in frames)
then TP's Force operator should have a value of about -98.1
Void Sphere and Void Cylinder
Void Sphere and Void Cylinder only work on moving particles! If you use those forces on static particles you won't see any effect.