Discovering the Shape of a Bending Plate: Mathematical Proof in 2D Statics

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    2d Bending Plate
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Discussion Overview

The discussion revolves around the shape that a plate or beam takes when supported in the middle and bending under its own weight. Participants explore the mathematical proof of this shape, considering different geometrical and material properties, and the implications of various assumptions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that the shape of the beam could be either an inverted parabola or a catenary, but expresses uncertainty about how to mathematically prove this.
  • Another participant questions the initial assumptions, asking for clarification on whether it is a beam or a plate, its geometry, and material properties.
  • A catenary shape is proposed to be relevant for a flexible beam supported at both ends, while a parabolic shape is suggested to imply a specific relationship between vertical deflection and distance from the support.
  • One participant mentions the principle of least action as a potential method for proving the shape.
  • Another participant describes a scenario where the beam is treated as a half-length beam bolted to a wall, indicating that this could provide insight into the problem.
  • Clarification is provided that in both the full beam and the half-length beam scenarios, there is no net moment at the support, and the two cases are considered physically identical under certain conditions.

Areas of Agreement / Disagreement

Participants express differing views on the shape of the beam and the assumptions necessary for analysis. The discussion remains unresolved, with multiple competing ideas and no consensus reached on the correct approach or shape.

Contextual Notes

The problem is noted to be under-specified, with limitations related to missing assumptions and the dependence on definitions of the beam and plate. The implications of material properties and geometry are also highlighted as factors that could influence the analysis.

ucsbphysics
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This is sort of statics, but this came up in my physics class before.

You have a plate, supported only in the middle by a simple support. So, this beam is balanced on the support, and is bending under it's own weight. What shape does the beam take? I was thinking either a inverted parabola or catenary shape? But I don't know how to prove this.

How can you show mathematically that this is the shape it has?
 
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Welcome to PF;
You have a plate, supported only in the middle by a simple support. So, this beam is balanced on the support, and is bending under it's own weight.
Is it a beam or a plate? What is it's geometry? What are it's material properties?

Continuing for a beam (metal? rectangular?):
What shape does the beam take? I was thinking either a inverted parabola or catenary shape?
Just on intuition?

A catenary would follow for a flexible beam supported at each end.
A parabola would imply that the vertical deflection is proportional to the square of the distance from the support.

But I don't know how to prove this.
Have you tried an application of the principle of least action?

For a beam where width W and height H are: W,H<<L, then try treating it as a half-length beam bolted to a wall at one end. What shape does that make?

Bottom line - the problem is under-specified.
Have you tried looking it up?
 
A plate with length L, and yes it is assumed that L>>H. It could be metal. Just has thickness, H and density, rho. That is just on intuition. Could you say something about the half-length beam attached to a wall because I see how you can treat that the same. The difference being the full beam doesn't have moment at the support.

Thank you!
 
In the full beam, the extra moment at the support is provided by the other half of the beam.
There is no net moment at the support in either case.
The two cases are physically identical provided the horizontal width is too small to have significant distortion.
 

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