What causes the bending of this rod?

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Discussion Overview

The discussion centers around the bending of a rod subjected to angular acceleration at its base. Participants explore the forces involved in this bending, particularly questioning the role of inertial forces and internal moments. The scope includes conceptual understanding and mathematical reasoning related to mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that the bending of the rod may be due to a fictitious inertial force, questioning the nature of these forces and their effects on the rod's deformation.
  • Another participant explains that the rod bends to develop the internal moment necessary for the upper portion to achieve angular acceleration, indicating a relationship between bending and rotational motion.
  • A request for clarification on the concept of internal moment is made, indicating a need for deeper understanding of the mechanics involved.
  • A detailed explanation is provided about analyzing the rod as a free body, including the identification of external forces and internal forces/moments that act on different sections of the rod.
  • Participants discuss the calculation of axial force, shear force, and bending moment along the rod, noting the complexity of these calculations, especially for a flexible rod.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the forces at play, particularly the role of inertial forces and internal moments. There is no consensus on the exact nature of the forces causing the bending, and the discussion remains unresolved on some technical aspects.

Contextual Notes

The discussion highlights the complexity of calculating internal forces in a bending rod, with participants acknowledging that the motion of the rod complicates the analysis. Specific assumptions about the rod's material properties and the exact nature of the applied forces are not fully addressed.

jerronimo3000
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In this video, a man applies an angular acceleration to the base of a rod. While the rod rotates, it bends. Why? What force is there that causes the bending, aside from rod's own weight? It seems to me to be the work of a fictitious inertial force. I was always taught that those forces don't actually do anything, yet the rod is clearly deforming. Am I missing some other force, or am I misunderstanding inertial forces? Also, in this simple case, how would one go about computing the stress at the base of the rod?



Edit: Here's the same video, without the lines and data:

 
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It bends in order to develop the internal moment required to cause the upper portion to have an angular acceleration. If there were no bending, there would be no internal moment, and the upper part would not rotate.
 
Could you elaborate on internal moment? I'm not sure I'm grasping what you mean.
 
Imagine that you cut the rod at a distance s from the bottom end. On the bottom end, you have external forces creating a moment on the rod. Treat this lower section of the rod as an isolated free body. On that FBD, show the external forces/moments dues to the users hands, and on the upper end show (1) an axial force, (2) a shear force, and (3) a bending moment. These represent the forces and moment transferred up to move the upper part of the rod. These are called internal forces and moment. They are internal to the rod considered as a whole, but external to the FBD of the lower part only. Note that s can be anywhere, that is, the cut can be anywhere along the length of the rod. The values of the forces and moment vary with s, but they exist everywhere.
 
Ah, I think I understand better. Thank you.

How does one calculate the axial force, shear force, and bending moment at each point along the rod?
 
For the free body as I described it previously, you apply F = M*a and T = Ic *alpha and solve for the internal forces. It can be tricky, and not the easiest thing to explain. It is particularly awkward for something bendy like this fly rod where the motion is difficult to describe. It is much easier to get internal forces after you know the motion of the rod.
 

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