KnockBack Projectile

How exactly could I add knockback to a projectile?

PHP:

     public function onDamage(EntityDamageEvent $event){
        $player = $event->getEntity();
        $entity = $event->getEntity();
        if($player instanceof Player && $event->getCause() === EntityDamageEvent::CAUSE_PROJECTILE){
           $event->setDamage(6);
           $event->setKnockBack($event->getKnockBack() * 2);
           }
      }
}      

No, I mean have the projectile launched even further after its been spawned

Disable or lower gravity for arrow.

Level::addEntityMotion()?

Either that, or reduce the drag.