void SchQ_Particle_System :: apply_hooks_law(Spring &s){
Vec2D<double> pos0 = s.a->get_position(), pos1 = s.b->get_position(),
vel0 = s.a->get_velocity(), vel1 = s.b->get_velocity();
double dx = pos0.x-pos1.x, dy = pos0.y-pos1.y,
dvdx = vel0.x-vel1.x, dvdy = vel0.y-vel1.y,
dist = pos0.distance(pos1), rdist = dist != 0 ? 1.0/dist : 0;
Vec2D<double> force (-(s.stiffness*(dist-s.rest_length) +
s.dampening*(dvdx) * dx*rdist) * dx*rdist ,
-(s.stiffness*(dist-s.rest_length) +
s.dampening*(dvdy) * dy*rdist) * dy*rdist);
s.a->add_force(force);
s.b->add_force(force.multiply(-1));
}
