Example 7-10 Lagrangian Dynamics Marion and Thornton

Click For Summary

Homework Help Overview

The discussion revolves around the application of Lagrangian dynamics to a particle of mass m situated on a frictionless hemisphere. The original poster seeks to understand the generalized coordinates involved and the conditions under which the particle detaches from the hemisphere.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster questions why φ is not considered a proper generalized coordinate, suggesting that the particle should be able to move in all spherical coordinates (r, θ, φ). They also raise a constraint equation related to the radius of the hemisphere.
  • Participants discuss the implications of the Euler-Lagrange equations for φ and the complexities arising from the dependence of θ on time.
  • One participant reflects on the conservation of angular momentum and its implications for the motion of the particle.

Discussion Status

The discussion is active, with participants exploring various interpretations of the equations and concepts involved. Some have provided insights into the conservation of angular momentum, while others are clarifying their understanding of the generalized coordinates and the resulting equations of motion.

Contextual Notes

There is mention of a specific example from a textbook, and participants are navigating the complexities of the equations without reaching a definitive conclusion. The original poster expresses a desire for clarification on a related problem, indicating ongoing exploration of the topic.

MARX
Messages
49
Reaction score
1

Homework Statement


A particle of mass m is on top of a frictionless hemisphere centered at the origin with radius a"
Set up the lagrange equatinos determine the constraint force and the point at which the particle detaches from the hemisphere

Homework Equations


L=T-U

The Attempt at a Solution


this is NOT HW
solution is there in full (example from book) I am just trying to understand
why is φ not one of the proper generalized coordinates? can't the particle move sideways as well when released? shouldn't the GC be (r,θ,φ) ie all spherical and the constraint equation r-a=0.
THanks
 
Physics news on Phys.org
What would the Euler-Lagrange equations give you for ##\phi## ?
 
(sin(2φ)/2)*(dθ/dt)-(d^2/dt^2)(φ)=0
I though of integrating it but θ is also dependent on t
 
Last edited:
even if I replace dθ/dt from what I get from euler-L of θ I would still get cosθ in the above equation!
 
(Sorry for the late reaction) ##\quad##I find, with $$ \ T = {1\over 2} m\dot r^2+r^2\dot\theta^2+r^2\sin^2\theta\,\dot\phi^2 \ $$ (here, top of p7) and $$V = -mr\cos\theta\ $$ that $$ {\partial {\mathcal L}\over \partial \phi} = 0 $$ so that $${ d\over dt } 2r^2 \sin^2\theta\,\dot\phi = 0 \ ,$$in other words: ##L_z=## constant (##L## being the angular momentum). The particle is let go with ##L_z=0## so it stays at that value.
 
yes absolutely simple one line answer
forgive my stupidity I copied the T term from Wolfram Alpha and there they use θ for φ and vice vera- (I was too lazy to do this trivial math considering I'm a fanatic I solve every single example and problem in every chapter)
damn mathematicians-lol(kidding love them)
that explains why my differential equation above gets complicated and doesn't reveal conservation
I completely get it the angular momentum does not change in that direction
Lz is explicitly determined and happens to be a constant multiple of the euler-Lagrangian variation and that's why it never moves sideways I get it supports intuition in this case
Thank you so much for your help and clarification appreciated A+++++++
 
OK, on to the next ...
 
Sure
I am on problem 7-20 so far able to solve them all
actually I do have a question (clarification) on problem 7-10 should I post here or different thread
THanks
 

Similar threads

  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 11 ·
Replies
11
Views
3K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 3 ·
Replies
3
Views
3K
Replies
3
Views
6K
  • · Replies 2 ·
Replies
2
Views
5K
  • · Replies 4 ·
Replies
4
Views
9K
  • · Replies 1 ·
Replies
1
Views
2K
Replies
3
Views
3K
  • · Replies 17 ·
Replies
17
Views
4K