An body m is suspended by a string from the center of the ceiling of a passenger car. If passenger car is accelerated to the right, string and body will tilt downward to the left. But, what if passenger car is jerk-moving? The inclination of string and body will continue to increase, the tension in string will increase, and eventually the string will break.
These situations and explanations of the situation must be the same for one inside and outside passenger car. Inertial force is not a fictitious force for one inside and outside the car. Inertial force in this case is action, and the reaction is the tension in string.
