NAME:
phiforce
PURPOSE:
evaluate the azimuthal force F_phi (R,z,phi,t)
INPUT:
R - Cylindrical Galactocentric radius
z - vertical height
phi - azimuth (rad)
t - time (optional)
OUTPUT:
F_phi (R,z,phi,t)
HISTORY:
2010-07-10 - Written - Bovy (NYU)