Pressure in an inverted cylinder partially filled with water

[zerogoal]

Hi may i know if the pressure of air in the cylinder is acting upwards or downwards? I know the pressure of the cylinder is acting downwards and the pressure of water is acting upwards.

My teacher gave me this equation Pcyclinder= -(Pair + Pwater)

However i do not know why Pair is acting in the same direction as Pwater. I thought Pair should be acting downwards, acting on the water instead?

Can someone help me with this? Thanks

 

[Xerxes1986]

P_air is the pressure of the air INSIDE the cylinder. you basically have 2 forces acting on the plate, up and down. The "up" force is atmospheric pressure that you can calculate knowing the pressure and the surface area. The "down" force is actually the sum of 3 forces. The weight of the plate, the weight of the water (pressure of the water), and the pressure, P_air, of the air inside the cylinder.

 

[zerogoal]

hey thanks for taking the time to help me with this question. finally understood.