Physics question II10

According to Newtons First Law of Motion (Inertia), it states that a body will remain at rest or travel in uniform velocity unless it is acted by a net force. When the car accelerates, the balloon will tend to stay at its original position and its original speed, i.e. stationary.
(Since the only friction between the helium-filled balloon and the fixed components inside the car is a string, the balloon will stay stationary until the string has tension to pull the balloon (e.g. when the length from the tied end of the string to the balloon increases, tension increases), or until the balloon hits fixed components in the car.)
Therefore, the balloon is moving relatively backwards to the car since it is not accelerating together with the car.