If you have a car and a pendulum is hanged inside it, and if you watch from outside the car,and if the car has some acceleration, then you see the bob moving in opposite direction of motion of car. For a person standing outside, he knows that the car has some acceleration. What causes the bob to move backwards from his point of view? its not pseudo force because the frame of reference is inertial. the car accelerates and so does tge bob. But why in opposite direction? Also if you have a person inside the car, since bob is at rest, he introducea the concept of psuedo force to balance tension and weight. Am I right? How do you explain first case?