Why Is My Fluid Mechanics Solution Incorrect?

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Discussion Overview

The discussion revolves around a fluid mechanics homework problem involving the calculation of pressure and forces acting on a cylinder submerged in water. Participants explore concepts related to pressure at different depths, the direction of forces on the cylinder's surfaces, and the calculation of resultant forces, including the force on the curved surface of the cylinder.

Discussion Character

  • Homework-related
  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant expresses uncertainty about their solution and seeks clarification on what is incorrect.
  • Another participant questions the pressure at depth and the direction of pressure acting on the cylinder's surfaces.
  • There is a discussion about the equation for pressure, p = po + ρgz, and the meaning of po.
  • Participants calculate pressures at different depths but express confusion about units and the correct interpretation of forces.
  • There is a debate about the direction of pressure forces on the cylinder's end faces, with differing opinions on whether they are parallel or perpendicular to the cylinder's axis.
  • One participant suggests using Archimedes' principle to determine the total force on the cylinder, while another expresses difficulty with integral calculations.
  • Participants discuss the weight of the displaced water and its relationship to the upward force on the cylinder, leading to confusion about mass and weight.
  • There is a calculation of the weight of the displaced water, with one participant initially misunderstanding the relationship between mass and weight.

Areas of Agreement / Disagreement

Participants do not reach a consensus on several points, including the correct interpretation of pressure forces and the calculations related to the cylinder's submerged conditions. There are multiple competing views on the direction of forces and the methods for calculating resultant forces.

Contextual Notes

Participants express uncertainty about units and the application of fluid mechanics principles, indicating potential gaps in understanding. There are unresolved issues regarding the calculations of forces and the distinction between mass and weight.

Who May Find This Useful

This discussion may be useful for students studying fluid mechanics, particularly those grappling with concepts of pressure, force calculations, and the application of Archimedes' principle in practical scenarios.

  • #31
Hi iSimon,

I really don't know how to answer you. The original diagram of the system was very ambiguous, and we took our best shot at trying to figure out what it was indicating. But, if we guessed wrong, then we were solving the wrong problem. The region of the figure that is most confusing is the left hand side where the water just seems to end at a slanted plane. I don't know whether this is a solid boundary, or what.

From your rendering of your teacher's explanation, it isn't clear to me what she is saying.

What i am confident about is that our solution to the problem as we interpreted it is correct. If it were not correct, we wouldn't have satisfied the check that the force on the curved surface is perpendicular to the axis.

I would be glad to help you some more if we could get some clarification on the figure.

Chet
 
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  • #32
Here is the main idea on how to solve.

jUPq9BJ.jpg


If we transpose nodes of the cylinder to the right side we will receive figure which will be composed from 2 identical elipses on top and bottom and two identical rectangulars in middle.

wuaUNb5.jpg


And the thing is to add Areas of separate parts multiplied by center of gravity. Sum them up and receive force which will act on horizontal axis in the right direction. But I am a bit lost in trygonometry to find proper values to calculate areas of elipse and rectangulars. Maybe This time I;ve managed to think better, I spoke to my teacher and she says its good solution but I need to calculate it.
 
  • #33
Sorry. I still don't understand the geometry.

Chet
 

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