Hydro static forces on curved surfaces

[princejan7]

Homework Statement

http://postimg.org/image/4fhu5k9r9/

Can someone explain what is meant by 'missing water' in this solution diagram
The original question diagram had no such water above the gate AB

Homework Equations

The Attempt at a Solution

 

[SteamKing]

Homework Statement

http://postimg.org/image/4fhu5k9r9/

Can someone explain what is meant by 'missing water' in this solution diagram
The original question diagram had no such water above the gate AB

Homework Equations

The Attempt at a Solution

By Pascal's Law, the hydrostatic pressure on the wetted portion of the arc AB at a given depth h from the free surface of the liquid is the same as the pressure exerted by a column of liquid of depth h placed on the outside of the arc.

If the region indicated by 'missing water' on the diagram was filled with water, there would be no net pressure acting on the arc AB, i.e., the pressure outside the arc would be the same as the pressure inside the tank.