Lagrangian mechanics - rotating rod

AI Thread Summary
The discussion centers on understanding the kinetic energy of a rotating rod, specifically why it includes both translational and rotational components. It highlights that when calculating kinetic energy, the frame of reference affects the consideration of these components, particularly when the rod is fixed at one end. The conversation emphasizes the importance of recognizing the center of mass and how forces like gravity act on the entire mass distribution. A simple beam attached at one end serves as a practical example to illustrate these concepts. Overall, the discussion aims to clarify the physical intuition behind the mathematical formulation of kinetic energy in such systems.
pj33
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Hello,

It might sound silly, but when I try to calculate the kinetic energy of a rotating rod to form the Langrangian (and in general), why it has both translational and rotational kinetic energy?

Is it because when I consider the moment of Inertia about the centre I need to include the translational since my "frame of reference" is the centre and it moves but when considering about end I only take into aacount the rotatioal since by "frame of reference" (end of rod) is stationary?

I am looking more for a physical interpretation/intuition.
I hope my explanation above is clear enough!

Thank you in advance!
 
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You talk about a stationary end of rod, so you must have some specific configuration in mind. Can you divulge to us what it is ?

Note that ##\int {1\over 2} {\dot r}^2 \; dm\ ## is the kinetic energy in all cases. Often it is convenient/ useful/ practical to split it up in parts, e.g. as in your c.o.m plus rotation. A force like gravity works on all ##dm## but can comfortably be considered to work on the c.o.m. that way.

All in all nothing physical, just a mathematical approach :wink:

##\ ##
 
BvU said:
You talk about a stationary end of rod, so you must have some specific configuration in mind. Can you divulge to us what it is ?

Note that ##\int {1\over 2} {\dot r}^2 \; dm\ ## is the kinetic energy in all cases. Often it is convenient/ useful/ practical to split it up in parts, e.g. as in your c.o.m plus rotation. A force like gravity works on all ##dm## but can comfortably be considered to work on the c.o.m. that way.

All in all nothing physical, just a mathematical approach :wink:

##\ ##
I was just thinking of a simple beam attached at one end at a stationary point.
If I understand this, it helps to tackle harder problems. Thank you!
 
pj33 said:
I was just thinking of a simple beam attached at one end at a stationary point.

That is a system with a constraint, so already not all that trivial, but surely instructive.

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This has been discussed many times on PF, and will likely come up again, so the video might come handy. Previous threads: https://www.physicsforums.com/threads/is-a-treadmill-incline-just-a-marketing-gimmick.937725/ https://www.physicsforums.com/threads/work-done-running-on-an-inclined-treadmill.927825/ https://www.physicsforums.com/threads/how-do-we-calculate-the-energy-we-used-to-do-something.1052162/
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