pressure
For the balloon, if the thing is tied then the internal pressure acts equally in all directions on the inner surface. Open the back end and there is an unequal distribution. Same with rocket engine ie its the net force obtained by the pressure in the chamber. Only force causes acceleration but conservation of momentum or energy often gets the result easier, however it is always a net force.
In a turbojet, not sure but there is the bypass flow generating a differential pressure and the reaction force on the fan blades (as someone else said).
Had to do the pressure integration myself a generation ago, for a rocket.
That was at Caltech, btw. Applied Physics. Doesn't make me infallible however.