cle to get N/m The force is applied to all the surface particles. The spring force is calculated based on the difference from the particle's initial position. Only for 3D applications <a class="el" href="class_s_p_h_1_1_body_part_by_particle.html" title="A body part with a collection of particles. ">BodyPartByParticle</a> define the ody part that the spring is applied to. Only for uniform surface particle size.  <a href="class_s_p_h_1_1solid__dynamics_1_1_spring_on_surface_particles.html#details">More...</a