/**
* @function gravitateTo
* @param {Particle} particle
* @memberof Particle
*/
public gravitateTo(particle: Particle) {
var grav = new Vector(0, 0)
var distance = this.position.distanceTo(particle.position)
grav.setLength(particle.mass / (distance * distance))
grav.setAngle(this.position.angleTo(particle.position))
this.velocity.add(grav)
}