Final answer:
The plastic bag flew out of the car window because of a pressure differential created when the window was rolled down, causing the still air inside to rush out towards the lower pressure outside.
Step-by-step explanation:
When David rolled down the window of his car while driving on the freeway, the empty plastic bag on the floor flew out due to a concept known as air pressure differential. The air inside the car was relatively still compared to the fast-moving air outside when the window was opened. This sudden change caused a pressure difference between the inside and outside of the car. As a result, the air inside, seeking to balance with the outside pressure, rushed out, carrying with it any light objects like the plastic bag. This phenomenon is related to Bernoulli's principle, which states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.