PPT Slide
What are the dynamics of a falling film?
Think of the film emerging as a skydiver out of a plane. As she jumps she quickly accelerates, and so does the film. As the film spreads out, air drag across its ever expanding surface becomes important. The film quits accelerating, and may even slow down, as is the case with the parachutist as she pulls open the chute. Once the film reaches the parallel section of the channel, its speed will be constant, as it is for the skydiver, who now glides down to earth.