Pendulum consists of a rod of mass m attached to a light rod

AI Thread Summary
The discussion focuses on deriving the equations of motion for a pendulum system consisting of a uniform rod and a light rod. The user attempts to apply the Lagrangian method, defining the coordinates of the center of mass and the relevant kinetic and potential energies. There is confusion regarding the moment of inertia (I) and angular velocity (ω) for the rod of mass m, with the user unsure if their initial choices for these parameters were correct. The user seeks clarification on how to properly select I and ω to successfully apply the Euler-Lagrange equations. The thread highlights the complexities involved in modeling the dynamics of pendulum systems using Lagrangian mechanics.
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Homework Statement


A pendulum consists of a uniform rod of mass m and length l hanging from the bottom end of a light rod of length l which top end is fixed to the ceiling. (see file attached)
System moves in a vertical plane. Find equations of motion.

Coordinates of the center of mass (X,Y)
angles θ and ψ of the light rod and the rod of mass m with the vertical respectively.

Homework Equations



Lagrangian method

L=T-U
U=mgY
T=mV2/2 + IΩ2/2

V is the velocity of the center of mass respect to a system at rest which origin is the top end of the light rod.

The Attempt at a Solution



X=l/2sinψ + lsinθ
Y=-l/2cosψ -lcosθ

|V|2= l2/4\dot{ψ} + l2\dot{θ}2 + l2\dot{ψ}\dot{θ}cos(ψ-θ)

I relative to the top end of the rod of mass m I=ml2/3
ω=\dot{ψ}

then i will plug this into L= T-U and the fin the Euler- Lagrange equations.
but i am not sure about I and ω.

I am confused. My first attempt was to choose same X,Y,V but I relative to the center of the rod of mass m I= ml2/2 and ω=\dot{ψ}+\dot{θ}
 

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