Do you need a passport when you get on a airplane and travel to another state?
You NEED a passport to travel outside the country. For domestic travel, no passport is necessary. It can be convenient to have a passport when flying, even within the country. The airline agents and the airport security people tend not to hassle people with passports.
You can fly between any of the 50 states without needing a passport. You can drive between the contiguous 48 states without needing a passport but do need a passport to drive to Alaska because you have to drive through Canada.
No. You only need it if you're traveling internationally.
no another country or across an ocean
no because your still in the country