Radial force due to the spiral arm in the direction ϕ = 0. The circles in red represent the radial force due to our potential, while the crosses in green represent the radial force associated with the classical sine function perturbation. Here, positive values of the force mean that a star would be pushed outward and negatives values that it would be pulled inward with respect to the galactic center. In this figure, the number of arms is m = 2, the pitch angle i = 14°, ε_s = 0.4 kpc^-1, and for our model the arm width σ = 4.7 kpc.