# Why water rises up in a straw when we blow over it?

Suppose I am holding a straw in a cup filled with water and I blow over the straw, the water rises up. The explanation I have seen anywhere on the internet is "Bernoulli's principle" i.e the air is moving at higher speed on top of straw than the air inside the straw creating a lower pressure which moves water upwards (from high to low pressure) but this seems so flawed because Bernoulli principle is applicable for a single flow at a time right? Why it's being applied at two different flows here? I see people using it everywhere like to explain the airplane lift also but it works with one flow at a time why it's been used here?

• Who tells this is related to Bernoulli is right. If you're familiar with a bit of fluid mechanics at university level, I can try to provide you an answer woth some more detail and formula, and general approach to this kind of problems Commented Dec 18, 2023 at 14:21
• I am in high school, and my textbook says it is related to Bernoulli's principle. But Bernoulli principle is applicable to a SINGLE streamline flow of fluid, that's what we derived it it from. How can we compare two different systems here using Bernoulli principle?. Sure we can give a formula saying the air above is moving but air in straw is still so the pressure difference would be =1/2(rho)v^2, but we compared two different systems here isn't it? Commented Dec 18, 2023 at 16:41

You are not completely correct when saying that Bernoulli's principle is applicable only for "single streamline flow" at a given,the Bernoulli principle in standard high school form relates the velocity,pressure and potential (gravitational) of a fluid,in the given example as long as we are aware of the pressure difference at two points and the height of the straw,density of water we can apply Bernoulli's principle for water too at any given instant in case we assume streamline flow which is almost true,also to clear up confusion air in the staw first moves up,i.e. air in the straw is first evacuated then the water moves up.

But we particularly don't need Bernoulli's principle to explain as to why the water moves up the straw,its just due to the atmosphere pushing the water up due to the pressure at upper end of the straw being less than the atmospheric pressure.

As the air over the straw moves at a higher velocity than surrounding air,it creates a region of low pressure compared to the surrounding in accordance with Bernoulli's principle,Now since the water at surface is exposed to atmosphere and the atmospheric pressure is comparatively greater than that at the upper end of straw,this causes the atmosphere to push water up the straw.

• " as long as we are aware of the pressure difference at two points and the height of the straw,density of water we can apply Bernoulli's principle" So once we know the pressure, we can use Bernoulli's principle to find the pressure? "But we particularly don't need Bernoulli's principle to explain as to why the water moves up the straw,its just due to the atmosphere pushing the water up due to the pressure at upper end of the straw being less than the atmospheric pressure." But why is the pressure different? y? Commented Dec 25, 2023 at 5:42
• "As the air over the straw moves at a higher velocity than surrounding air,it creates a region of low pressure compared to the surrounding in accordance with Bernoulli's principle," Why? Commented Dec 25, 2023 at 5:42
• I assumed the author of question is aware of the Bernoulli principle as they state so. Also the pressure decreases in accordance with Bernoulli's principle because Bernoulli's principle is basically a set of conditions/constraints which need to be satisfied in case we have a streamline flow,to understand it fully you need to see its derivation.
– Blz
Commented Dec 25, 2023 at 6:06
• The author explains why they think Bernoulli's principle doesn't apply. You spent three paragraphs not answering that question. Commented Dec 25, 2023 at 6:09
• It rather seems to me that the author is more concerned as to how can we apply it both for the flow of water up tbe straw and the air moving above the staw simultaneously.
– Blz
Commented Dec 25, 2023 at 6:10