% drag_force: The force of drag on a sphere.
%
% F=drag_force(V,R,rho,T,A,[Z]);
%
% ARGUMENTS
%  V	Velocity
%  rho	Density of gas
%  T	Temperature of gas [K]
%  A	Mean atomic mass of gas
%  Z	Mean charge of gas particle (optional)
%
% RETURNS
%  Force due to drag, in units of rho*V^2*R^2
%
% USES
%  Reynolds_number, drag_coef
%
% SEE ALSO
%  dynamic_viscosity, impact_paramenter

% AUTHOR: Eric Tittley
%
% HISTORY
%  06 03 01 Standardized header comments.

function F=drag_force(V,R,rho,T,A,Z)
if(nargin==5) % neutral gas
 Re=Reynolds_number(R,V,rho,T,A);
elseif(nargin==6)
 Re=Reynolds_number(R,V,rho,T,A,Z);
end
Cd=drag_coef(Re);
F = 0.5*Cd*rho*V^2*pi*R^2;