Archive for Lagrangian Mechanics

A Problem in Lagrangian Mechanics

Posted in Cute Problems with tags , , on April 28, 2016 by telescoper

Today, as well as saying goodbye to Sally Church, I managed to finish my lecture course on Theoretical Physics. There’s still another week of teaching to go, but I have covered all the syllabus now and can use the remaining sessions for revision. The last bit of the course module concerned the calculus of variations and a brief introduction to Lagrangian mechanics so for a bit of fun I included this example.

Professor Percy Poindexter of the University of Neasden has invented a new theory of mechanics in which the one-dimensional motion of a particle in a potential V(x) is governed by a Lagrangian of the form

L=mx\ddot{x} +2V(x).

Use Hamilton’s Principle and an appropriate form of the Euler equation to derive the equation of motion for such a particle and comment on your answer.

UPDATE: Since nobody has commented I’ll just reveal the point of this question, which is that if you follow the instructions the equation of motion you should obtain is

m\ddot{x}= -\frac{\partial V}{\partial x},

which is exactly the same as you would have got using the usual Lagrangian

L= \frac{1}{2}m\dot{x}^{2} - V(x).

Anyone care to comment on that?