# Why can we assume that Velocity at the top of large tank is zero?

When using Bernoulli's Equation we sometimes assume that the velocity is zero if the tank is very large. Why can we do this? Is it because the pressure is assumed to be zero as well?

• It's not zero, but insignificant.
– Mast
Sep 18, 2020 at 8:45

$$V_{1}A_{1}=V_{2}A_{2}$$
Which says the volume flow rate is constant (fluid is neither piling up or being depleted within the tank). So the larger the cross sectional area $$A$$, say at the top of the tank, the lower the flow velocity $$V$$ to maintain the same volume flow rate as at the lower cross sectional area at the tank exit. If $$A_1$$>>$$A_2$$, then $$V_1$$<<$$V_2$$ and can be neglected.