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While calculating the velocity of efflux, i.e., the velocity of a liquid when it flows from a orifice which is made in a beaker filled with liquid. Since at the orifice, atmospheric pressure acts along with the the hydrostatic pressure, why don't we consider the opposing effect of atmospheric pressure?

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    $\begingroup$ @Spirko has the right answer. If you cap the beaker, then as the liquid level begins to drop the air pressure above the fluid will begin to drop. Then the fluid flow out the orifice will slow as air bubbles begin to force their way in. $\endgroup$ – James Dec 1 '15 at 13:01
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Atmospheric pressure acts on the top surface of the fluid and also on the exiting fluid. It is only the pressure difference (caused by the depth of the fluid) that determines the flow.

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