dsolve({diff(diff(x(t),t),t) = -C(y(t))*f((diff(y(t),t)^2)^(1/2))/(diff(y(t),t)^2)^(1/2)*diff(x(t),t), diff(diff(y(t),t),t) = -C(y(t))*f((diff(y(t),t)^2)^(1/2))/(diff(y(t),t)^2)^(1/2)*diff(y(t),t)-g})