I'd like to drop an object weighing 1-5 pounds and use my action to cast Catapult on the item. PH 190 details Other Activity on your turn, and interacting with an object is included on that list. Can I use this Other Activity to remove a one pound rock from my pocket and drop it on the ground in front of me?
You can pull something out of your pocket then catapult it
The rules for Interacting with Objects Around You gives pulling something out of your backpack, or a pouch as valid examples.
Dropping something is easy, so easy that the lead designer believes it takes no effort.
So yes, you can pull out a rock, drop it, and can cast catapult on it.