acceleration, just for the sake of easier calculations.

(A d'Alembert force is not to be confused with a contact force arising from the physical interaction between two objects, which is the subject of Newton's third law - 'action is reaction'. In terms of the example of the passenger vehicle above, a contact force emerges when the body of the passenger touches the backrest of the seat in the car. It is present for as long as the car is accelerated.)