Apparent forces are forces that only appear for an observer within an accelerating reference frame and are not subject to Newton's third law. They arise from the observer's inertia and are therefore also called inertial forces. An intuitive example is the force acting on passengers in a moving bus. The passengers feel an invisible force pushing them against their seats. However, there is no opposing force to this force. An outside observer would correctly interpret the situation: As the bus moves, the seats press against the passengers. Since the passengers have inertia due to their mass, a force is required to accelerate the passengers. This creates an equal opposing force on the seats, which is the only force observed by the passengers. Therefore, when passengers are present, the bus must exert a greater force to achieve the same acceleration, as its total mass increases.
A similar situation applies to the centrifugal force, which can only be measured by observers moving in a circular orbit. A well-known example is the rotational motion in a centrifuge used in astronaut training. The observer moving with the centrifuge is pushed outward by an apparent force, while an outsider describes the situation as follows: The centrifuge seat must exert a centripetal force on the occupant to force them into a circular orbit, whereas the occupant exerts an opposing force on the seat.
A third apparent force also arises in rotating reference frames and is called the Coriolis force. For example, if two people stand directly opposite each other on a rotating disc and throw a ball to each other, the ball appears to be deflected in the opposite direction to the disc's rotation. Here, too, the situation can only be correctly described from the outside: After being thrown, the ball continues to move in a straight line due to its inertia, as it is no longer in contact with the disc, while the people on the disc continue to move continuously in the direction of rotation. A famous example of the Coriolis force is Foucault's pendulum, which can be used to demonstrate the Earth's rotation. While the pendulum plane appears to rotate to an observer on Earth, it remains constant to an observer from space, whereas the observers on Earth move around the pendulum. Therefore, choosing the right reference system is essential for correctly describing movements.