285 A gate having the shape shown in Fig. P2.85 is located in the vertical side of an open tank containing water. The gate is mounted on a horizontal shaft. (a) When the water level is at the top of the gate, determine the magnitude of the fluid force on the rectangular portion of the gate above the shaft and the magnitude of the fluid force on the semicircular portion of the gate below the shaft. (b) For this same fluid depth determine the moment of the force acting on the semicircular portion of the gate with respect to an axis that coincides with the shaft.

Hello. Our instructor solved this question and subtracted the product of water's height and specific weight from the P1 pressure, as well as oil's. He didn't factor air pressure into the equation. What is the reason why we do not use open air pressure to solve the problem?

2.78 An automobile has just dropped into a river. The car door is approximately a rectangle, measures 36 in. wide and 40 in. high, and hinges on a vertical side. The water level inside the car is up to the midheight of the door, and the air inside the car is at atmospheric pressure. Calculate the force required to open the door if the force is applied 24 in. from the hinge line. See Fig. P2.78. (The driver did not have the presence of mind to open the window to escape.) 36" -24" V 40 ■FIGURE P2.78