$6-21$ A $90$ elbow in a horizontal pipe is used to direct water flow upward at a rate of $40k\frac{g}{s}.$ The diameter of the entire elbow is $10$ cm $.$ The elbow discharges water into the amosphere, and thus the pressure at the exit is the local amospheric pressure. The elevation difference between the centers of the exit and the inlet of the elbow is $50$ cm $.$ The