What causes the bending of this rod?

[jerronimo3000]

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:

 

[Dr.D]

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.

 

[jerronimo3000]

Could you elaborate on internal moment? I'm not sure I'm grasping what you mean.

 

[Dr.D]

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.

 

[jerronimo3000]

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?

 

[Dr.D]

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.