Another interesting bit on that subject.
If you want to make a trip combat maneuver, do you have to use a weapon with the trip special feature?
No. Note that when making a trip combat maneuver, you don't need to use a weapon at all--for example, you can trip when you're unarmed, even though unarmed strike isn't listed as a trip weapon.
There are advantages to using a weapon with the trip special feature (a.k.a. a "trip weapon") when making a trip combat maneuver.
One, if your trip attack fails by 10 or more, you can drop the trip weapon instead of being knocked prone.
Two, you can apply the weapon's enhancement bonus, weapon-specific attack bonuses such as Weapon Focus, and so on to your trip combat maneuver roll.
For example, you'd add the enhancement bonus from a
+5 whip to your trip combat maneuver roll because a whip is a trip weapon. You wouldn't add the enhancement bonus from a
+5 longsword to your trip combat maneuver roll because a longsword is not a trip weapon. In effect, there's no difference between making an unarmed trip attempt and a trip attempt with a
+5 longsword because the sword doesn't help you make the trip attempt.
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Source]
—Sean K Reynolds, 03/15/11
Basically, trip attacks are considered modified unarmed attacks as far as bonuses to CMB are concerned; only certain weapons allow you to add any bonuses derived from weapons.