function F = F_roll(m, v)
%   F_ROLL mass, velocity
%   F_ROLL(M, V) computes the force necessary to move a
%   vehicle of mass M in kilograms travelling at
%   velocity V in meters per second.

error(nargchk(2, 2, nargin));

g = 9.81;
F = c_R(v) * m * g;