Buoyancy Force question, having hard time starting.

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Discussion Overview

The discussion revolves around a homework problem involving the calculation of the angle theta for a plank submerged in water, focusing on concepts of buoyancy and forces acting on the plank. The scope includes mathematical reasoning and conceptual clarification related to buoyancy forces and equilibrium conditions.

Discussion Character

  • Homework-related, Mathematical reasoning, Conceptual clarification

Main Points Raised

  • One participant expresses confusion about the dimensions to consider for the plank, questioning whether to account for its thickness and how to use the density of the plank in relation to the density of water.
  • Another participant suggests that the water pressure force acting on the plank needs to be considered, alongside the weight of the plank and the buoyant force at the centroid of the submerged section.
  • A later reply indicates that the participant believes the water pressure may not be a separate factor in the problem, suggesting it could be included in the buoyancy calculation.
  • One participant confirms that they arrived at the correct answer as provided in the textbook.
  • Another participant acknowledges the progress made by the first participant and expresses support for their solution.

Areas of Agreement / Disagreement

The discussion shows some initial uncertainty regarding the factors to consider in the problem, particularly about the role of water pressure. However, one participant ultimately confirms their answer aligns with the textbook, indicating a resolution for their specific query, while other aspects of the discussion remain less clear.

Contextual Notes

Participants do not fully resolve the implications of water pressure versus buoyancy in their calculations, leaving some assumptions about the problem's setup and the definitions of forces involved unaddressed.

frozenguy
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Homework Statement


The 3m plank shown in section has a density of 800 kg/m3 and is hinged about a horizontal axis though its upper edge O. Calculate the angle theta assumed by the plank with the horizontal for the level of fresh water shown.

Homework Equations


Whats wrong with the editor? It isn't working properly for me at all.
B=rho*g*V
Sum of MO=0


The Attempt at a Solution


I bolded in section because I was wondering what that meant exactly. Like I should only consider 1 meter in width? What about its thickness? Am I just supposed to use density of the plank to compare to density of water like in a ratio of some sorts?
I was thinking I need to find that distance from the centroid of buoyancy to the water line so I can find that length and then find theta.. But I'm not to sure on where to go.
statics5199.jpg
 
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Ok so I've been thinking this over more and I realized I need to take into account the water pressure force acting on the plank. So I have mg and the center of gravity, I have B at the centroid of the section under water, and I have the force of water pressure on the submerged part.

Sum around O=0 should give me my answer ya?

EDIT: I guess the water pressure is NOT a factor in this problem, or its included in the buoyancy.
 
Last edited:
Got it! The answer checks with the one provided from the book.
statics5199-1.jpg
 
Good job. I clicked into your thread yesterday, but didn't have time to be of help. Glad that you figured it out on your own!
 

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