Final answer:
The sensation of being pushed back into the seat during takeoff in a jet aircraft is caused by a backward force or thrust.
Step-by-step explanation:
When you take off in a jet aircraft, the sensation of being pushed back into the seat is due to a force called backward force or thrust. According to Newton's third law of motion, for every action, there is an equal and opposite reaction. In this case, when the jet engines produce a forward thrust, there is an equal and opposite backward force exerted on the occupants of the aircraft, pushing them back into their seats.