What is "making the time integration redundant"?

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Discussion Overview

The discussion revolves around the phrase "making the time integration redundant" in the context of variational principles in mechanics, particularly focusing on nonholonomic constraints and their implications in deriving equations of motion. Participants explore the relationship between different equations presented in a referenced document and the mathematical implications of these relationships.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the meaning of "making the time integration redundant" and suggests that setting ##t_2=t_1## leads to a trivial equation.
  • Another participant proposes that equation (28) can be derived from equation (50) by differentiating with respect to time, which they argue avoids the need for integration.
  • A different participant expresses confusion over the complexity of nonholonomic constraints in textbooks, suggesting that they should be viewed as constraints on "allowed variations" rather than restrictions to a submanifold.
  • A later reply elaborates on the previous points, indicating that when the endpoints are fixed, the variation ##\delta q_j## is zero at those points, leading to a specific relationship between ##\delta L## and ##Q_j^{NP}##.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of nonholonomic constraints and the derivation of equations, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

There are unresolved assumptions regarding the definitions and implications of nonholonomic constraints, as well as the mathematical steps involved in the derivations discussed.

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What does it mean by "making the time integration redundant" (5th line)? If I let ##t_2=t_1##, I will only get ##0=0-0##.

Source: http://www.phys.ufl.edu/~maslov/classmech/flannery.pdf
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I don't pretend to know what half of this is about, but it looks to me that (28) can be got from (50) by differentiating wrt time. That certainly avoids the integration.
 
haruspex said:
I don't pretend to know what half of this is about, but it looks to me that (28) can be got from (50) by differentiating wrt time. That certainly avoids the integration.

I think I figured it out.

##\delta q_j=0## when ##t=t_1## and ##t=t_2## since the end points are fixed as we vary the trajectory.

That leaves us with ##\int_{t_1}^{t_2}\delta L\,dt = -\int_{t_1}^{t_2}Q_j^{NP}\delta q_j\,dt##.

By "making the time integration redundant", we have ##\delta L = -Q_j^{NP}\delta q_j##.

Then by expressing ##\delta L## in terms of ##\delta q_j##, we obtain (14) and subsequently (28).
 

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