void OnCollisionEnter(Collision collision)
{
Instantiate(soundEffect, collision.transform.position, Quaternion.identity);
GravityObject collisionGravObj = collision.gameObject.GetComponent <GravityObject>();
if (collisionGravObj.GetID() < GetID())
{
AddMass(collisionGravObj.GetMass());
Destroy(collision.gameObject);
}
}