Problem with Lagrange first kind equations

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In summary, the problem involves two masses moving in concentric circles with a solid rod of length B connecting them. The Lagrange first order equations are used to find the equilibrium point, with the constraint given by B = R1^2 + R2^2 - 2R1R2cos(θ1 + θ2). The Lagrangian is written as L = (m/2)(R1^2ω1^2 + R2^2ω2^2) - mg(sinθ1 + sinθ2), and the equations of motion are found to be -mgcos(θ1) + mR1^2α1 + R1R2sin(θ1 - θ2)
  • #1
carllacan
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Homework Statement


Two masses move in a plane restricted to concentric circles with radii R1 and R2. They are joined by a solid rod of length B. Use Lagrange first order equations to find the equilibrium point


Homework Equations


Constraint due to the solid bar: B = R12 + R22 -2R1R2cos(θ1 + θ2), where θ1 and θ2 are the polar coordinates of the masses.

The Attempt at a Solution


My Langrangian is as follows: L = (m/2)(R12ω12+R22ω22)-mg(sinθ1+sinθ2)
By substituing in this expression http://en.wikipedia.org/wiki/Lagrangian_mechanics#Lagrange_equations_of_the_first_kind I find two equations of motion:
-mgcos(θ1)+mR12+α1 + R1R2sin(θ1 - θ2)λ1 = 0
-mgcos(θ2)+mR22+α2 - R1R2sin(θ1 - θ2)λ2 = 0
where α1 and α2 are the angular accelerations of the masses, and the λs are the Lagrange multipliers. I can't solve this equations, though. Is there any method of solving them that I cannot find or is it that my whole procedure is wrong?
 
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  • #2
Can you show us what your Lagrangian looks like, that you use to derive these equations of motion?
You denoted the constraint by ##L##, but I don't think that expression is your Lagrangian because I don't see a kinetic term.
 
  • #3
Sorry, its a bit confusing because the length of the bar is L. I'll change it to B and edit the rest.
 

Related to Problem with Lagrange first kind equations

1. What is the problem with Lagrange first kind equations?

The problem with Lagrange first kind equations is that they are not always solvable, as they can lead to singularities or infinite solutions.

2. How does the Lagrange first kind equation differ from the other kinds?

The Lagrange first kind equation is a form of constrained optimization, where the constraints are expressed directly in the objective function. This differs from the second and third kinds of Lagrange equations, which involve the use of Lagrange multipliers.

3. When are Lagrange first kind equations typically used?

Lagrange first kind equations are typically used in situations where the constraints are simple and can be easily incorporated into the objective function. They are commonly used in economics and physics, among other fields.

4. What are some common techniques for solving Lagrange first kind equations?

Some common techniques for solving Lagrange first kind equations include the method of undetermined multipliers, the substitution method, and the elimination method.

5. How do Lagrange first kind equations relate to the principle of least action?

Lagrange first kind equations are a mathematical formulation of the principle of least action in physics. This principle states that the path taken by a physical system between two points in time is the one that minimizes the action, which is a measure of the system's energy.

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