Fluid mechanics problem, need explanation please

Hi please check the highlighted question in the attachment and the solution and see the problem, i need help please. I have two questions about it:

1- why didn't we take the plug as a vertical wall and thus the force would be integral of pressure x area?

2-WHERE DID THE FORCE DUE TO ATMOSPHERIC PRESSURE GO?

Thnx for your help.

Attached Images

	Screen Shot 2013-08-27 at 9.48.23 PM.jpg (135.3 KB)
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