Bernoulli’s Equation for plugging finger into dike under water

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The discussion centers on the application of Bernoulli's equation to a problem involving water flow through a hole in a dike. Participants debate the assumption that water outside the hole is stationary, questioning the validity of setting v_1 to zero. It is noted that while the pressure near the hole may vary, moving further away allows for simplifications that yield consistent results. The pressure at the water surface is confirmed to be atmospheric, while the pressure at the hole's level is higher due to the water column. Ultimately, the conversation clarifies how to effectively choose points for applying Bernoulli's principle in this context.
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Homework Statement
A legendary Dutch boy saved Holland by plugging a hole
of diameter 1.20 cm in a dike with his finger. If the hole was 2.00 m below the surface of the North Sea (density 1030 kg/m^3) (b) If he pulled his finger out of the hole, during what time interval would the released water fill 1 acre of land to a depth of 1 ft? Assume the hole remained constant in size.
Relevant Equations
Bernoulli’s Equation
For this part (b) of this problem
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,
The solution is,
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However, why are they allowed to assume for the Bernoulli's equation for the water outside the hole, that the water is stationary (i.e v_1 = 0)?

It also appears that they assume that the pressure inside the Holland is 1 atm, which is only valid when the water level is below the hole.

Many thanks!
 
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Callumnc1 said:
, why are they allowed to assume for the Bernoulli's equation for the water outside the hole, that the water is stationary (i.e v_1 = 0)?
On the sea side, I assume you mean.
You are right that immediately in front of the hole the water will be moving some, but also the pressure will have dropped a little in consequence. As you move a bit further back into the water, both of those effects diminish.
You could also forget about that region and instead consider the water surface. That certainly gives v1=0, z1=2m, p1=atm.
 
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haruspex said:
On the sea side, I assume you mean.
You are right that immediately in front of the hole the water will be moving some, but also the pressure will have dropped a little in consequence. As you move a bit further back into the water, both of those effects diminish.
You could also forget about that region and instead consider the water surface. That certainly gives v1=0, z1=2m, p1=atm.
Thanks for your reply @haruspex ! I was assuming Holland was 2 m below the water as the problem says that. Were you assuming Holland was at sea level?

Is that why you said that I would be allowed to forget about that region and instead consider the water surface? The water surface is at 1 atm, while the region at the same height as the hole (2m below the surface) is at 1 atm + 20.2 kPa.

Many thanks!
 
Callumnc1 said:
Were you assuming Holland was at sea level?
No.
In using Bernoull's principle you pick any two points in the 'streamline'. You have the exit point on the Holland side of the hole, but on the water side you do not have to choose a point right in front of the hole. For the reasons I gave, it is not entirely clear what the velocity and pressure are there. Instead, you can choose a point somewhat further away, on the same level, or a point on the surface. The first will give a pressure patm+gρz (z=2m) and a height difference zero, the second will give a pressure patm and a height difference z=2m. Either way, it adds up to the same.
 
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haruspex said:
No.
In using Bernoull's principle you pick any two points in the 'streamline'. You have the exit point on the Holland side of the hole, but on the water side you do not have to choose a point right in front of the hole. For the reasons I gave, it is not entirely clear what the velocity and pressure are there. Instead, you can choose a point somewhat further away, on the same level, or a point on the surface. The first will give a pressure patm+gρz (z=2m) and a height difference zero, the second will give a pressure patm and a height difference z=2m. Either way, it adds up to the same.
Oooh thanks so much @haruspex , I understand now!
 
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