do they have a PVector in the direction in that they fly?

I am using Verletphysics with Vec2D.
I also tried to use math operations in vectors. But I was not succesful.

I added the script below. If you run it, you'll see that the line does not change at all and points only in one direction.
The part that it is drawn is marked in bold.

1. import toxi.physics2d.behaviors.*;
   import toxi.physics2d.*;
   import toxi.geom.*;
   import toxi.math.*;
   
   VerletPhysics2D physics;
   Cluster cluster;
   
   
   void setup() {
     size(1280, 800);
     smooth();
   
     //INITIALIZE PHYSICS
     physics=new VerletPhysics2D();
     physics.setWorldBounds(new Rect(0,0,width,height));
     physics.setDrag(0.1);
     //INITIALIZE CLUSTER
     cluster = new Cluster(10, 0.000001, 30);
   }
   
   
   void draw() {
     //UPDATE PHYSICS
     physics.update();
     background(255);
     cluster.display();
     cluster.showConnections();
   }
   class Attractor extends VerletParticle2D {
   
     float r;
   
     Attractor(Vec2D loc) {
       super(loc);
       r = 10;
       physics.addParticle(this);
       physics.addBehavior(new AttractionBehavior(this, 500, 0.2));//RADIUS AND STRENGTH
     }
   
     void display () {
       fill(100);
       ellipse (x, y, r*2, r*2);
     }
   }
   class Particle extends VerletParticle2D {
   
   
     float r;
     Particle(Vec2D loc) {
       super(loc);
       r=10;
       physics.addParticle(this);
       physics.addBehavior(new AttractionBehavior(this, r*4, -1));
     }
   
     void display() {
       fill(0);
       ellipse(x, y, r, r);
     }
   }
   class Cluster {
   
     ArrayList <Particle> particles;
     ArrayList <Attractor> attractors = new ArrayList <Attractor>();
     float diameter;
     float springRadius = 40;
     float springForce;
     int gridDimension;
   
     Cluster(float d, float sf, int gridDim) {
       particles = new ArrayList<Particle>();
       diameter = d;
       springForce = sf;
       gridDimension = gridDim;
       int w = width/gridDim;
       int h = height/gridDim;
   
       //ARRAYLIST OF PARTICLES
       for (int y = 0; y<gridDim; y++) {
         for (int x = 0; x<gridDim; x++) {
           particles.add(new Particle(new Vec2D(  (x+0.5)*w, (y+0.5)*h  )));
         }
       }
   
       //INITIALIZE ATTRACTOR CLASS
       attractors.add( new Attractor(new Vec2D(200, 200)));
       attractors.add( new Attractor(new Vec2D(900, 500)));
   
       //ARRAYLIST OF SPRINGS PHYSICS
       for (int i = 0; i<particles.size()-1; i++) {
         VerletParticle2D ni = particles.get(i);
         for (int j = i+1; j<particles.size(); j++) {
           VerletParticle2D nj = particles.get(j);
           if (dist(ni.x, ni.y, nj.x, nj.y)<springRadius) {
             //physics.addSpring(new VerletSpring2D(ni, nj, d, sf));//FIZIKA LINIJU SPRINGU, VISO SPRINGU YRA 3
           }
         }
       }
     }
   
   
     void display() {
       for (Attractor a: attractors) {
         a.display();
         for (Particle p: particles) {
           p.display();
           a.lock();
         }
       }
     }
   
     void showConnections() {
       stroke(0);
       strokeWeight(2);
       for (int i = 0; i<particles.size()-1; i++) {
         VerletParticle2D pi = (VerletParticle2D) particles.get(i);
         for (int j = i+1; j<particles.size();j++) {
           VerletParticle2D pj = (VerletParticle2D) particles.get(j);
   
           if (dist(pi.x, pi.y, pj.x, pj.y)<springRadius) {
   
             //line(pi.x, pi.y, pj.x, pj.y);
             line(pi.x, pi.y, pi.x+10, pi.y+10);
             noFill();
             // ellipse(pi.x, pi.y,10,10);
           }
         }
       }
     }
   }
   
   

Also, I think directionality should be in Particle class... But it is only my guess...

FOUND:))

 for (VerletParticle2D p : physics.particles) {
      noStroke();
      ellipse(p.x, p.y, 20, 20);
      stroke(0);
      Vec2D vel=p.getVelocity().getNormalizedTo(5);
      line(p.x,p.y,p.x+vel.x,p.y+vel.y);
    }

AS I SAW THIS POST:

https://forum.processing.org/topic/particleconstraint-usage

Reminder: when you ask a question involving or about a contributed library, post it in this section, instead of Programming Questions (I moved it).