Hydrostatic forces on a submerged curved surface question

In summary: How would you find the area of the shape above A, I am asking as i tried subtracting A from the area of the entire semi circle, but in order to find B we need to take into account the area above/next to...
  • #1
abuh11
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No template because of misplaced homework
Hi, i would like to know how to do this question for fluid mechanics. Sorry as I am new i didnt know how to upload images so i just uploaded the image in justpaste, here is the link http://justpaste.it/pz7r
 
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  • #2
May you know a pressure definition like ##P=\frac{F}{A}##.
You can use it as ##dF = P\,dA## with direction vertical to surface.
 
  • #3
theodoros.mihos said:
May you know a pressure definition like ##P=\frac{F}{A}##.
You can use it as ##dF = P\,dA## with direction vertical to surface.
I think what you mean is "perpendicular to the surface" rather than "vertical to the surface." Abuh11, to elaborate, the pressure at any depth is acting in the direction perpendicular to the surface. So you need to determine the pressure at each depth, multiply by the differential area involved, and add the forces vectorially (i.e., integrate over the arc of the surface).
 
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  • #4
Chestermiller said:
I think what you mean is "perpendicular to the surface" rather than "vertical to the surface." Abuh11, to elaborate, the pressure at any depth is acting in the direction perpendicular to the surface. So you need to determine the pressure at each depth, multiply by the differential area involved, and add the forces vectorially (i.e., integrate over the arc of the surface).

Thanks for your reply, what I normally do is use F = pgha to find horizontal component of the force, then use F = phv to find vertical component forces, then I find out the actual force using F^2 = FH^2 + FV^2. However the problem I’m having is finding the vertical component as I don’t know how to calculate the volume of the water that the shape is occupying. I normally work with questions in which the curved surface is fully submerged whereas here it is only partially submerged. The correct answer is meant to be 52050N although I’m not getting it with my current calculations.
 
  • #5
There's another way to do this. Do you know how to calculate the weight of the part of the water sitting directly above the gate?
 
  • #6
Chestermiller said:
There's another way to do this. Do you know how to calculate the weight of the part of the water sitting directly above the gate?

no
 
  • #7
abuh11 said:
no
Do you know how to find the volume of this water using geometry or calculus?
 
  • #8
Chestermiller said:
Do you know how to find the volume of this water using geometry or calculus?

I used geometry but not calculus
 
  • #9
abuh11 said:
I used geometry but not calculus
OK. From geometry, what is the volume of water directly under the gate.
 
  • #10
Chestermiller said:
There's another way to do this. Do you know how to calculate the weight of the part of the water sitting directly above the gate?

That is part of the problem, i know how to do this for when the curved surfaces is fully submerged, but for this type of question i don't know how to do it.
 
  • #11
abuh11 said:
That is part of the problem, i know how to do this for when the curved surfaces is fully submerged, but for this type of question i don't know how to do it.
I asked you for the volume of liquid below the gate. This is a geometry question, not a physics question. This is just the first step in getting to the answer you want. If you 're not willing to be patient, I won't be able to help you. This is part of how we determine the vertical component of force on the gate.
 
  • #12
Chestermiller said:
I asked you for the volume of liquid below the gate. This is a geometry question, not a physics question. This is just the first step in getting to the answer you want. If you 're not willing to be patient, I won't be able to help you. This is part of how we determine the vertical component of force on the gate.

i don't know how to do that for this type of question.
 
  • #13
Hint: Find area A, then area B.
 

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  • #14
insightful said:
Hint: Find area A, then area B.

Interesting, i see where you are going with this, but how would you find the area of B.
 
  • #15
You find A +B and subtract A to get B
 
  • #16
Chestermiller said:
You find A +B and subtract A to get B
Ok, i done that. I got A and B now, what should i do with these numbers now?
 
  • #17
abuh11 said:
Ok, i done that. I got A and B now, what should i do with these numbers now?
Area B can get you to the volume of water displaced, right?
 
  • #18
insightful said:
Area B can get you to the volume of water displaced, right?

How would you find the area of the shape above A, I am asking as i tried subtracting A from the area of the entire semi circle, but in order to find B we need to take into account the area above/next to A.
 
  • #19
abuh11 said:
...in order to find B we need to take into account the area above/next to A.
Not really. What are the angles of triangle A? What is the area of the sector of the circle (A+B)?
 
  • #20
insightful said:
Not really. What are the angles of triangle A? What is the area of the sector of the circle (A+B)?

I don't think we know the angles of A.
 
  • #21
abuh11 said:
I don't think we know the angles of A.
Hmm...a right triangle with two of the sides known...you've had trigonometry, right?
 
  • #22
insightful said:
Hmm...a right triangle with two of the sides known...you've had trigonometry, right?
yay. I manged to figure it out using your diagram and information. Thanks a lot.
 
  • #23
abuh11 said:
yay. I manged to figure it out using your diagram and information. Thanks a lot.
An interesting problem; I enjoyed working on it.
 
Last edited:
  • #25
insightful said:
I like problems with obvious real-world application:

https://en.wikipedia.org/wiki/Tainter_gate

Yeah this question was actually one question from a exam for my course, I am studying civil engineering (one of benefits as you mentioned is that the math is applied).
 

1. How is the hydrostatic force on a submerged curved surface calculated?

The hydrostatic force on a submerged curved surface is calculated by multiplying the pressure at a given depth by the projected area of the curved surface. The pressure at a given depth can be calculated using the equation P = ρgh, where P is pressure, ρ is the density of the fluid, g is the acceleration due to gravity, and h is the depth of the fluid.

2. What factors affect the hydrostatic force on a submerged curved surface?

The hydrostatic force on a submerged curved surface is affected by the density of the fluid, the acceleration due to gravity, and the shape and orientation of the curved surface. The depth of the fluid and the shape of the curved surface also play a role in determining the magnitude and direction of the force.

3. How does the orientation of a curved surface affect the hydrostatic force?

The orientation of a curved surface can greatly affect the hydrostatic force. If the curved surface is horizontal, the force will be perpendicular to the surface. If the curved surface is vertical, the force will be parallel to the surface. The orientation of the curved surface can also affect the magnitude of the force, as a more angled surface will have a larger projected area and therefore a larger force.

4. What is the significance of hydrostatic forces on submerged curved surfaces?

Hydrostatic forces on submerged curved surfaces are important to understand in fields such as fluid mechanics, engineering, and hydrology. They play a crucial role in determining the stability and equilibrium of structures, such as dams, ships, and submarines. Understanding these forces is also important in designing and constructing hydraulic systems, such as pipes, pumps, and turbines.

5. Can the hydrostatic force on a submerged curved surface be negative?

Yes, the hydrostatic force on a submerged curved surface can be negative. This occurs when the curved surface is oriented in such a way that the force is acting in the opposite direction of the applied pressure. This can happen, for example, when the curved surface is concave and the pressure is acting from the outside of the curve. In this case, the force will be negative and will push the curved surface inwards.

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