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

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:bust_in_silhouette: Asked By NoctilucaScintillans

the rigid body rotates when I moving it on a surface using add_central_force(). however, according to my knowledge of physics, it should not rotate. the document also says add_central_force() 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))

enter image description here

:bust_in_silhouette: Reply From: whiteshampoo

Because the box and the gound have friction.