Suppose that a small ball is hung at the roof of a car, through a string. The car is accelerating forward and we see that the ball moves backward giving an angle to the string. In the point of view of an observer inside the car, what balances the horizontal component of the stress of the string? I know that we can say ball moves back due to a pseudo force (since the observer is in a non-inertial frame), but can we call this force as "inertial force"? is that the correct term? Further, according to an observer outside the car, what is the force that causes the ball to move back? is it again inertial force?
A simple explanation would be enough.