Finding Lagrangian for Overhanging String on Frictionless Table

In summary, the rope falls off the table within 1 second in the case of α=0 and 5 seconds in the case of α=1.
  • #1
Boltzman Oscillation
233
26

Homework Statement


B is 10kg
C is 20kg
can I find a lagrangian for this system? If so how?

Diagram: http://imgur.com/j811rzw
j811rzw

Homework Equations


L=T-V
Kinetic = .5mv^2
Potential = mgh

The Attempt at a Solution


I know the kinetic energy must be 0 right? How could I find the potential?
 
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  • #2
Hi Boltzmann Oscillation and welcome to PF.

Yes, the kinetic energy is zero. If you take the zero of potential energy to be at the same height as the lights, the potential energy will also be zero, so the Lagrangaian is zero. May I ask why you are looking for the Lagrangian in relation to a statics problem?
 
  • #3
kuruman said:
Hi Boltzmann Oscillation and welcome to PF.

Yes, the kinetic energy is zero. If you take the zero of potential energy to be at the same height as the lights, the potential energy will also be zero, so the Lagrangaian is zero. May I ask why you are looking for the Lagrangian in relation to a statics problem?
I have statics class and this was assigned but I am quite bored with statics. I want to apply what I've learned in calculus and physics courses. Lagrangian are only used for moving objects? Thank you so much for the help.
 
  • #4
Boltzman Oscillation said:
Lagrangian are only used for moving objects? Thank you so much for the help.
Lagrangians are used to find equations of motion and if something is not moving, well ...

If you want a diversion from statics and SOH-CAH-TOA, try this simple problem using a Lagrangian:
A uniform supple string of length ##L## and mass ##m## is held on a frictionless table with one of its ends overhanging the edge of the table. The overhanging length is ##\alpha L## (##\alpha<1##). The mass is released at time t = 0. Find the time it takes for the rope to fall off the table. Check your answer in the limiting cases ##\alpha=0## and ##\alpha=1##.
 

1. What is a Lagrangian?

A Lagrangian is a mathematical function used in classical mechanics to describe the dynamics of a physical system. It is named after Joseph-Louis Lagrange, a renowned mathematician and physicist.

2. How do I know if a system has a Lagrangian?

A system can have a Lagrangian if it follows the principles of conservation of energy and momentum, and if it can be described using generalized coordinates. In simpler terms, if a system's energy and momentum can be conserved and its motion can be described using coordinates other than the usual Cartesian coordinates, then it has a Lagrangian.

3. Where can I find a Lagrangian for a specific system?

There are a few ways to find a Lagrangian for a specific system. One way is to use the Lagrangian formulation of classical mechanics, which involves writing the system's kinetic and potential energy in terms of generalized coordinates. Another way is to use the Hamiltonian formulation, which involves finding the system's Hamiltonian function and then taking its Legendre transform.

4. Can a Lagrangian be derived from a given set of equations?

Yes, a Lagrangian can be derived from a given set of equations. This process is known as the inverse problem of Lagrangian mechanics and it involves solving the Euler-Lagrange equations for the system's Lagrangian.

5. Is there a universal method for finding a Lagrangian?

No, there is no universal method for finding a Lagrangian. The process can vary depending on the complexity of the system and the chosen approach (Lagrangian or Hamiltonian formulation). It may require a combination of mathematical techniques and physical intuition to determine the correct Lagrangian for a given system.

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