Spring String example from coding challenge

Gallery
1

sketch

//f spring = -k*x
//f = ma
//m= -k
//a = x
float y = 250;
int restLength = 2;
//PVector velocity = new PVector(0,0);;
PVector force = new PVector(0,0);
float x = 0.0;
Particle bob = new Particle(350,300);
Particle anchor = new Particle(300,0);
Spring spring = new Spring(0.01,200,bob,anchor);
PVector gravity = new PVector(0,0.02);
PVector v = new PVector(0,0);
//spring constant
float k = 0.01;
float damping = 0.5;
float spacing = 10;
ArrayList<Particle>particles = new ArrayList<Particle>();
ArrayList<Spring>springs = new ArrayList<Spring>();
int particlesNum = 20;
void setup(){
  size(600,400);
  for(int i=0;i<particlesNum;i++){
    particles.add(new Particle(200,i*spacing));
    //particles.get(i).locked = true;
    if(i!=0){
      Particle a = particles.get(i);
      Particle b = particles.get(i-1);
      Spring spring = new Spring(k,spacing,a,b);
      springs.add(spring);
    }
  }
  particles.get(0).locked = true;
};

void draw(){
  background(112,50,126);
  for(Spring s: springs){
    s.update();
    //s.show();
    
  }
  
  Particle tail = particles.get(particles.size()-1);
  Particle head = particles.get(0);
  beginShape();
  noFill();
  curveVertex(head.pos.x,head.pos.y);
  for(Particle p: particles){
    p.applyForce(gravity);
    p.update();
    //p.show();
    curveVertex(p.pos.x,p.pos.y);
  }
  curveVertex(tail.pos.x,tail.pos.y);
  endShape();
  if(mousePressed){
    tail.pos.set(mouseX,mouseY);
    tail.vel.set(0,0);
  }
};

spring

class Spring{
  float k,restLength;
  Particle a,b;
  Spring(float k,float rl,Particle a,Particle b){
    this.k = k;
    restLength = rl;
    this.a = a;
    this.b = b;
  };
  
  void update(){
    
    PVector force = PVector.sub(b.pos,a.pos);
    float x = force.mag()-restLength;
    force.normalize();
    force.mult(k*x);
    a.applyForce(force);
    force.mult(-1);
    b.applyForce(force);
  };
  
  void show(){
    strokeWeight(4);
    stroke(255);
    //line(a.pos.x,a.pos.y,b.pos.x,b.pos.y);
    
  };
  
  
};

particle

class Particle{
  PVector acc,vel,pos;
  float mass = 1.0,damp = 0.99;
  boolean locked = false;
  Particle(PVector pos){
    this.acc = new PVector(0,0,0);
    this.vel = new PVector(0,0,0);
    this.pos = pos.copy();
  };
  
  Particle(float x,float y){
    this.acc = new PVector(0,0,0);
    this.vel = new PVector(0,0,0);
    this.pos = new PVector(x,y);
  };
  
  void run(){
    this.update();
    this.show();
  };
  
  void applyForce(PVector force){
    PVector f = force.copy();
    f.div(this.mass);
    acc.add(f);
  }
  
  void update(){
    if(!locked){
      vel.mult(damp);
      vel.add(acc);
      pos.add(vel);
      acc.mult(0);
    }
  };
  
  void show(){
    stroke(200);
    strokeWeight(2);
    fill(127);
    ellipse(pos.x,pos.y,10,10);
  };
  
};

output
image