Can I be tripped by a flat-footed opponent?

[The Grandfather]

If I make an unarmed trip against a flat-footed (who does not have either Combat Reflexes or Uncanny Dodge), can I be knocked prone for failing the combat maneuver check by +10?

[silverhair2008]

Going by the Core Rules:

Core Rules wrote:

Trip

You can attempt to trip your opponent in place of a melee
attack. You can only trip an opponent who is no more
than one size category larger than you. If you do not have
the Improved Trip feat, or a similar ability, initiating a
trip provokes an attack of opportunity from the target of
your maneuver.
If your attack exceeds the target’s CMD, the target is
knocked prone. If your attack fails by 10 or more, you are
knocked prone instead. If the target has more than two legs,
add +2 to the DC of the combat maneuver attack roll for each
additional leg it has. Some creatures—such as oozes, creatures
without legs, and flying creatures—cannot be tripped.

Yes.

[Gray Eminence]

The Grandfather wrote:

If I make an unarmed trip against a flat-footed (who does not have either Combat Reflexes or Uncanny Dodge), can I be knocked prone for failing the combat maneuver check by +10?

Yes, since you fall prone because you made a disastrous attempt, not because your opponent was so good.