Q.A passenger sitting in a car that takes a sharp turn feels pushed outward, towards the door. Explain whether this outward push is a real force or a pseudo-force, name the force, and state the reference frame in which it must be introduced.

Described from the ground (an inertial frame), no real outward force acts on the passenger at all. By Newton's first law, the passenger's body, due to its own inertia, tends to keep moving in a straight line; it is the car -- specifically, the door or the seat, pushing on the passenger -- that supplies the real, inward-directed centripetal force needed to pull the passenger's body along the car's actual curved path. What the passenger experiences as an "outward push" against the door is, correctly understood in this frame, really the door pushing inward on the passenger, resisted by the passenger's own inertia -- there is no genuine outward force present at all in this description.\n\nThe situation looks different only if it is instead described from a rotating frame attached to the car itself -- the frame the passenger's own body, in a sense, actually experiences. Since this rotating frame is non-inertial, Newton's laws can only be applied within it formally by introducing an additional pseudo-force, the centrifugal force, of magnitude $m\omega^2 r$, directed radially outward, exactly balancing the real inward centripetal force so that the passenger (who is genuinely at rest relative to the car) can be treated as being in equilibrium within that frame. This centrifugal force is what a person analysing the situation *fro …

Describe the situation in both frames: in the ground (inertial) frame only a real inward centripetal force exists; in the car's rotating (non-inertial) frame, an additional outward centrifugal …

• Claiming the outward push is a real force acting on the passenger in the ordinary (ground) frame, rather than recognising it only appears when analysis switches to the car's own rotating frame. …