applyBehaviour(particle, time, index) {
    this.distanceVec.set(this.centerPoint.x - particle.p.x, this.centerPoint.y - particle.p.y);
    const distanceSq = this.distanceVec.lengthSq();

    if (distanceSq !== 0) {
      const distance = this.distanceVec.length();
      const factor = (this.force * time) / (distanceSq * distance);

      particle.v.x += factor * this.distanceVec.x;
      particle.v.y += factor * this.distanceVec.y;
    }
  }
Apply this behaviour for all particles every time

@method applyBehaviour
@memberof Proton#GravityWell
@instance

@param {Particle} particle
@param {Number} the integrate time 1/ms
@param {Int} the particle index