The observation that the balloon appears to move backward relative to the car when the car starts moving forward can be explained by the concept of inertia, which is described by Newton's First Law of Motion.
Inertia is the tendency of an object to resist changes in its state of motion. When the car accelerates forward, the air molecules inside the car also move with it, but the balloon, filled with a gas that is less dense than the surrounding air, tends to remain in its original position due to its inertia. As the car moves forward, the balloon lags behind because it is not being pushed forward by the same force that is acting on the denser air inside the car.
As a result, to an observer inside the car, it looks like the balloon is moving backward when in fact it is simply continuing in its original path while the car moves forward. This phenomenon demonstrates how objects with different densities respond differently to force when in motion.