function setup() {

  createCanvas(400, 400);

  myParticle = new Particle(width / 2, height / 2, 0,0);

}

​

function draw() {

  background(200);

​

  var dx = mouseX - myParticle.px;

  var dy = mouseY - myParticle.py;

  var dh2 = (dx * dx + dy * dy);

  var dh = sqrt(dh2);

  if (dh > 5.0) {

    var attraction = 40.0;

    if (mouseIsPressed){

      attraction *= -1;

    }

    var fx = attraction * dx / dh2;

    var fy = attraction * dy / dh2;

    myParticle.impel(fx,fy);

  }

  myParticle.update();

  myParticle.render();

}

​

//----------------------------------------

class Particle {

  constructor(inpx, inpy, invx, invy) {

    this.mass = 1.0;

    this.FRICTION = 0.95;

    this.px = inpx;

    this.py = inpy;

    this.vx = invx;

    this.vy = invy;

  }

​

  impel(fx, fy) {

    // Add an acceleration (a = F/m)

    this.vx += fx / this.mass;

    this.vy += fy / this.mass;

  }

​

  update() {

    // Euler integration

    this.vx *= this.FRICTION;

    this.vy *= this.FRICTION;

    this.px += this.vx;

    this.py += this.vy;

  }

​

  render() {

    ellipse(this.px, this.py, 40, 40);

  }

}