Why, before takeoff and before landing, the passenger must wear a seat belt.

When the plane has already taken off, it flies at a constant speed V and according to Newton’s first law, such an aircraft can be considered stationary, and passengers can move around the cabin calmly.

But during takeoff and landing, the plane moves at an accelerated rate, that is, with some acceleration a (during takeoff, directed along the movement of the aircraft, and when landing, against the movement). During such a movement, an additional force m * a acts on the passengers, which sharply change the trajectory of its movement, and even throw it out of the seat. Therefore, be sure to wear seat belts.