why rigidbody rotates when I move it using add_central_force() ?

Question

the rigid body rotates when I moving it on a surface using addcentralforce(). however, according to my knowledge of physics, it should not rotate. the document also says addcentralforce() adds a constant directional force without affecting rotation.
more detailed information can be found in the code and pictures below:

func _physics_process(_delta):
  if Input.is_action_pressed("ui_right"):
    add_central_force(Vector2(16, 0))

Answer 1

Because the box and the gound have friction.