An oil storage tank has a flat, horizontal, circular roof that is 77 ft in diameter. The tank has a vent to the atmosphere to allow air to move in and out as the tank is filled or emptied. The vent is plugged by snow in a blizzard while the oil is being pumped out of the tank and the gauge pressure in the tank falls to -.8 psig. What is the net force on the roof of the tank?