Spaceship is moving horizontally with uniform acceleration. Body m is pushing against rear wall of the spaceship's cabin (A). Rear wall pushes back the body with normal force (B). A is action, B is reaction.
Spaceship is moving horizontally with uniform acceleration. String attached to front wall of the spaceship's cabin stretches backward inside the cabin, pulling body m attached to rear end of string (A). The body resists pulling force of string (inertial resistance) (B). A is action, B is reaction.
To begin with, distinction between action and reaction seems to be unclear.
