Does a sine-shaped beam remain sine when pushed?

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Discussion Overview

The discussion centers on the behavior of a linear elastic thin beam described by the function y=sin(pi x) when subjected to a static load that compresses one end. Participants explore whether the beam's shape remains a sine wave after compression, considering the implications for calculating the force applied by the beam.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Homework-related

Main Points Raised

  • One participant questions whether the beam shape remains a sine wave after being pushed from x=1 to x=1-delta, given the constraints of the beam.
  • Another participant argues that due to the finite modulus and non-zero mass per unit length, the x/y relationship will not remain a sine wave during compression, as parts of the beam will compress at different rates.
  • A different viewpoint suggests that if small compressions can maintain the sine function, then larger compressions should also maintain it, although they acknowledge that large compressive forces could deform the beam significantly, particularly in the middle.
  • One participant clarifies that the inquiry is focused on a static load problem, not wave propagation, and seeks to understand the function for force calculations.
  • Another participant expresses agreement that the beam can be assumed to remain approximately sine if the deflection is small.

Areas of Agreement / Disagreement

Participants express differing views on whether the beam retains a sine shape after compression. While there is some agreement that small deflections may allow for an approximate sine shape, the overall discussion remains unresolved regarding the effects of larger compressions.

Contextual Notes

Participants note the importance of the static load context and the potential for deformation due to leverage effects, which may complicate the relationship between compression and the sine wave shape.

Who May Find This Useful

This discussion may be of interest to those studying mechanics of materials, structural engineering, or anyone involved in analyzing the behavior of elastic beams under load.

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I have a linear elastic thin beam y=sin(pi x) from 0<x<1 and the beam is pin supported (no moment applied) at x=0 and x=1 and constrained so that the ends remain on the x axis. Then I push the end from x=1 to x=1-delta (for some small delta, say 0.1). Will the resulting beam shape still be a sine?

Thanks
 
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It will have a finite modulus and non-zero mass per unit length so the speed of propagation of a compression wave will be finite, hence the x/y relationship will not be a sine wave over the length of the beam during the change because the nearer parts will have compressed before the far parts have moved, distorting the shape.
 
If a slight compression maintaines the sine function, then any compression, large or small should maintain the function since a large compression would equate to many small compressions.
However it is easy to visualize a large compressive force deforming the middle portion due to leverage much more than the sides eventually forcing the bottom to arc inwards. This can not be a sine wave, despite the possibility of a sine wave having infinitely small wave length.
 
In case anyone is confused I'm asking this as a static load problem, nothing with wave propagation.

I'm asking because I want to calculate the force applied by the beam when I compress it, and I need to know the function in order to calculate it.

a1call: Can I at least assume that the beam remains approximately sine if the deflection is small?
 
Last edited:
I would say yes.
 

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