Water flows through a pipe that gradually descends from a height of 6.78 m to the ground. near the top, the cross-sectional area is 0.4 m^2, and the pipe gradually widens so that its area near the ground is 0.8 m^2. water leaves the pipe at a speed of 16.8 m/s. what is the difference in the water pressure between the top and bottom of the pipe?