(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A massless spring (spring constant k) is attached to the ceiling. It is free to move only in the y-direction. A homogenous bar of mass m and

length l is attached from its other end to the lower end of the spring, and the bar is free to move on the xy-plane.

A fancy picture for those who appreciate:

+<--y-->-

|~~~--

|=ceiling, ~~~=spring, --=bar

Find the Lagrangian equations of the system.

2. Relevant equations

[tex]\frac{d}{dt}\frac{\partial L}{\partial \dot{q}_i}-\frac{\partial L}{\partial q_i}=0[/tex]

3. The attempt at a solution

I suppose there's no gravitational force present, as there's no mention of it, although the spring is attached to the ceiling. But this

problem still stuns me.

First, there's the spring related energy of the system.

[tex]L=T-V=\frac{1}{2}m\dot{y}^2-\frac{1}{2}ky^2[/tex]

However, in this case the velocity doesn't depend only on y (the location of the moving end of the spring), but it also depends on the

movement of the center of the bar, as it's free to rotate around the end of the spring. Rephrasing the previous:

The center of mass of the bar:

[tex]y_b=y+\frac{l}{2}cos\theta[/tex]

where theta is the angle away from the vertical

[tex]\dot{y_b}=\dot{y}-\frac{l}{2}\dot{\theta}sin\theta[/tex]

[tex]L=T-V=\frac{1}{2}m\dot{y_b}^2-\frac{1}{2}ky^2

=\frac{1}{2}m\left(\dot{y}-\frac{l}{2}\dot{\theta}sin\theta\right)^2-\frac{1}{2}ky^2[/tex]

Secondly, there's the kinetic energy of the bar. The spring is massless so it has no kinetic energy, so the kinetic energy in the previous

equation is the translational energy of the bar. Additionally there's rotational kinetic energy (R) but no potential energy.

[tex]L=R=\frac{1}{2}J\omega^2[/tex]

Here

[tex]J=\frac{1}{3}ml^2[/tex]

[tex]\omega=\dot{\theta}[/tex]

So [tex]L=(T+R)-V=\frac{1}{2}m\left(\dot{y}-\frac{l}{2}\dot{\theta}sin\theta\right)^2

+\frac{1}{2}J\dot{\theta}^2-\frac{1}{2}ky^2[/tex]

I've been told that there's an error, but I haven't been told where it is. Of course my equations look awful, but knowing that doesn't help.

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# Homework Help: Lagrangian mechanics: a spring and a bar

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