Final answer:
The empty plastic bag flew out of the car window due to decreased air pressure inside the car, a result of Bernoulli's principle stating that pressure of a fluid decreases as velocity increases.
Step-by-step explanation:
When David rolled down the window on his car while driving on the freeway, the empty plastic bag on the floor flew out because of decreased air pressure inside the car. This phenomenon occurs due to the concept of fluid dynamics where the movement of air outside the car is faster than inside, causing lower pressure near the opening. As the air pressure is higher within the car in comparison to the outside, there is a net force acting on objects inside the car, such as the plastic bag, pushing them towards the area of lower pressure, which in this case is outside the window.
Similar effects are observed in other situations involving rapid movement of fluids, for example, shower curtains pushing inward when the water is on, and cars veering towards trucks when passing at high speeds on a highway. All of these demonstrate the principle that a fluid's (like air) pressure decreases as its velocity increases, known as Bernoulli's principle.