ArrayList particles;

Particle a;

float h = 0.2; //new time step

void setup() {
 size(600, 600); 
 stroke(0);
 strokeWeight(3);
 fill(150);
 smooth();
 
 particles = new ArrayList();
 
 a = new Particle();
 a.location = new PVector(width/2f, height/2f);
 a.fixed = true;
 
 particles.add(a);
}

void draw() {
  background(255);
  for (int i = particles.size()-1; i >= 0; i--) {
    Particle p = (Particle)particles.get(i);
    // experiment w/ different friction coefficients
    applyDissipativeForce(p, 0.01);
    applyAttractiveForce(a, p, 5000f, 50f); 
    if(!p.exist()) {
      particles.remove(i);  
    }
  }  
}


void mouseDragged() {
    particles.add(new Particle()); 
} 

void applyDissipativeForce(Particle p, float friction) {
  PVector f = PVector.mult(p.velocity, -friction);
  p.applyForce(f);
}

void applyAttractiveForce(Particle a, Particle b, float strength, float minDistance) {
  PVector dir = PVector.sub(a.location, b.location);
  float d = dir.mag(); 
  if (d < minDistance) d = minDistance;
  dir.normalize(); 
  float force = (strength * a.mass * b.mass) / (d * d); 
  dir.mult(force);
  if (!b.fixed) b.applyForce(dir);
  if (!a.fixed) {
    dir.mult(-1f);
    a.applyForce(dir);
  }
}


public class Particle {
  PVector location;
  PVector velocity;
  PVector acceleration;
  float mass;
  boolean fixed;
  
  public Particle() {
      location = new PVector(mouseX, mouseY);
      //get velocity from direction and speed of mouse movement
      velocity = new PVector(mouseX-pmouseX, mouseY-pmouseY);   
      acceleration = new PVector(0, 0, 0);
      mass = 1;
      fixed = false;
  }
 
  public boolean exist() {
     if (fixed) {
       fill(255,0,0); 
     } else {
       velocity.add(PVector.mult(acceleration, h));
       location.add(PVector.mult(velocity, h));
       acceleration.mult(0);
       fill(150);
     }
     ellipse(location.x, location.y, 10, 10); 
     return true;
  }
  
  void applyForce(PVector force) {
      acceleration.add(PVector.div(force, mass));
  }
    
}