Forward movement, forward momentum. Newton's First Law: A body at rest stays at rest, and a body in motion stays in motion.
It's getting out of the gate that's always the challenge.
They close the door, have you buckle and watch the safety blarg, the Captain comes on and says what a super day to fly it is, we'll be cruising at Thirty Zillion Feet, and then you sit. And wait. And sit. And wait. And every 20 minutes the First Officer - because the Captain is (a) too smart to do it and (b) has the authority to delegate the dirty-work -- comes on "ahhh, from the cockpit," and says that we're just waiting for a plane to move out of the alley and to get our clearance from ground control and for a bit of paperwork from maintenance and for the baggage handler's cart to be moved and for the Wright Brothers to invent aviation, and that won't take but another 10 minutes or so, and then we'll be pushing back and on our way.
Which is the same thing he just said. 100 minutes ago.
And you get tenser and tenser and tenser. You start doing that "I'll glare up at the cockpit -- that'll make it go!" thing. The kids have already eaten all their snacks and colored all their books and the batteries on the PlayGameStationTendos are drained. You've looked at every every household gizmo you don't need in the Shopping In The Sky catalog and done the crossword and the sudoku and the cross-stitch in the In-Flight Horizons Of The Sky Things magazine and are trying to entertain Themselves with air-sickness-bag hand puppets and....
At last! Movement! Backwards at first, pulling away from the gate..... Now forwards. A bit. Then they push the throttles and...thrust. Inertia, overcome. More thrust. Accelerating now thumpity bumpity clumpity down the runway, a body in motion tending to stay in motion, and...... you're in the sky! The tension lifts. Everyone is happy again.
You're On Your Way.
(And you can start focusing on the mortal, elbow-to-elbow combat you have to wage with that whodoeshethinkheisso-and-so in the seat next to you for dictatorial control of the armrest.)
