I Why are there 2s -1 independent integrals of motion?

AI Thread Summary
The discussion revolves around the concept of independent integrals of motion in mechanics, specifically addressing confusion regarding the number of initial conditions required. It highlights that while 2N initial conditions are typically specified, the distinction between position and velocity is questioned, suggesting that they should be treated separately. The Lagrangian's independence from acceleration is emphasized, leading to a discussion about the implications for equations of motion. The need for time independence in autonomous systems is also examined, particularly regarding the relationship between time and position variables. Overall, the conversation seeks clarity on the mathematical foundations of motion integrals and their implications in mechanics.
yucheng
Messages
232
Reaction score
57
I was reading Mechanics by Landau and Lifshitz and I am confused when it is stated in chapter 2 section 6 that one of the integrals of motion is not independent and it can be considered an additive constant of time. Hence I tried searching it up online.

https://physics.stackexchange.com/q...f-motion-vs-first-integrals?noredirect=1&lq=1

According to the OP in the link above (first paragraph second sentence), we need to specify 2N initial conditions, one of them is the initial time, the others the initial positions and velocity.

However, shouldn't it be N position and N velocities? Can it be shown to be equivalent? Plus aren't we working with an autonomous system of equations? Is this why we need ##t - t_0## so that it is independent of time?

https://physics.stackexchange.com/questions/13832/integrals-of-motion

The answer provided above seems interesting. However, how correct is it? There are several points that I would like to verify...

Questions:
  1. Because the ##\mathcal{L} (q, \dot q)##, the Lagrangian is independent of the acceleration. Hence $$\frac{d}{dt} \frac{\partial \mathcal{L} }{\partial \dot q} - \frac{\partial \mathcal{L}}{\partial q}$$
    which only involved one time derivative, only introduces terms linear in ##\ddot q##.
  2. According to the author (see a comment below the post as well), $$\ddot q_i =\frac{\text{d}\dot q_i}{\text{d} q_1} \frac{\text{d}q_1}{\text{d}t} =\dot q_1\frac{\text{d}\dot q_i}{\text{d}q_1}$$. How does a total time derivative become a total derivative in ##q_1##? Are we performing a change of variables by inverting ##q_1(t)## to get time as a function of ##q_1## then all coordinates become ##q_i(t(q_1))##?
  3. Are there other proofs of this?
 
Last edited:
Physics news on Phys.org
yucheng said:
However, shouldn't it be N position and N velocities? Can it be shown to be equivalent? Plus aren't we working with an autonomous system of equations? Is this why we need t−t0 so that it is independent of time?
Hi. Position and velocity change in time in general so they cannot be integral constants e.g. energy.
 
anuttarasammyak said:
Hi. Position and velocity change in time in general so they cannot be integral constants e.g. energy.
Actually, for that part of the question, I am referring to the initial conditions (which of course determines the ##2s## integrals of motions). I asked it here because it is very relevant! ;)
 
Ooops by the way, ##N=s## (degrees of freedom)
 
yucheng said:
Because the L(q,q˙), the Lagrangian is independent of the acceleration. Hence $$\frac{d}{dt} \frac{\partial \mathcal{L} }{\partial \dot q} - \frac{\partial \mathcal{L}}{\partial q}$$
which only involved one time derivative, only introduces terms linear in q¨.
Is this correct?
 
Hello everyone, Consider the problem in which a car is told to travel at 30 km/h for L kilometers and then at 60 km/h for another L kilometers. Next, you are asked to determine the average speed. My question is: although we know that the average speed in this case is the harmonic mean of the two speeds, is it also possible to state that the average speed over this 2L-kilometer stretch can be obtained as a weighted average of the two speeds? Best regards, DaTario
This has been discussed many times on PF, and will likely come up again, so the video might come handy. Previous threads: https://www.physicsforums.com/threads/is-a-treadmill-incline-just-a-marketing-gimmick.937725/ https://www.physicsforums.com/threads/work-done-running-on-an-inclined-treadmill.927825/ https://www.physicsforums.com/threads/how-do-we-calculate-the-energy-we-used-to-do-something.1052162/
Back
Top