Torricelli's theorem relates the speed of a fluid exiting an opening in a reservoir to the height of the opening relative to the top of the reservoir... V=√2gh https://en.wikipedia.org/wiki/Torricelli's_law As seen in the wiki-link provided, the equation is essentially a Bernoulli's equation problem. My question is why the pressure at the opening where the fluid flows out is set as atmospheric pressure? Whenever you do a typical nozzle problem to find the speed in which a fluid exits a nozzle, you use the pressure on the inside of the nozzle, i.e. not atmospheric pressure. Why is this different?