Question

Argelia has a stack of schoolbooks sitting in the backseat of her car. When Argelia makes a sharp right turn, the books slide to the left of the seat until they come to a rest against the car door. Explain why this happens.

Answer 1

When the car turns right the books “want” to keep going in a straight line (Newton’s first law). From an outside (inertial) viewpoint the car is turning under the books; to make the books follow the curved path they need a rightward (centripetal) force supplied by friction from the seat or by the door. If that sideways force is too small the books do not get accelerated right enough and so move left relative to the car until they hit the left door.

In the car’s non‑inertial (turning) frame you can equivalently say a pseudoforce (often called the centrifugal force) appears to act outward — i.e. to the left during a right turn — and pushes the books against the left door. Either description is equivalent: inertia/cetripetal-force balance (inertial frame) or centrifugal pseudo‑force (car frame).

(If you want a quantitative condition: sliding occurs when the required centripetal acceleration v^2/r exceeds the maximum static friction a_max = μs g, i.e. v^2/r > μs g.)