Physics Help Forum why does the Lagrangian have this form?

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 Jul 28th 2017, 11:59 PM #1 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 534 why does the Lagrangian have this form? In this video set to play at the right position: It seems as if he comes up with the formula: $\displaystyle \delta A =\frac{\delta \mathcal{L} }{\delta x } - \frac{d}{dt} \frac{\delta \mathcal{L}}{\delta \dot{x}}$ However his justification seems to be that adding d/dt on the lagrangian with respect to velocity (x dot) is because velocity is calculated at different time intervals but then doesn't apply the same approach for the derivative of the lagrangian with respect to position. That is why does d/dt appear only once rather than for both terms?
 Jul 29th 2017, 05:37 AM #2 Physics Team   Join Date: Apr 2009 Location: Boston's North Shore Posts: 1,570 Yikes! That video is an hour and a half long! Too long for me to watch today. You could try to follow my own derivation here: Calculus of Variations I didn't like when he said that all "real" forces are derive by a potential function because its not true. The magnetic force can't be derived that way. In analytical mechanics (which this is) the magnetic force comes wat is known as a velocity dependent potential. Friction comes in when you use the Rayleigh dissipation function https://en.wikipedia.org/wiki/Raylei...ation_function Meanwhile I e-mailed Susskind and asked him your question - since I didn't watch the entire thing. Last edited by Pmb; Jul 29th 2017 at 05:54 AM.
 Jul 29th 2017, 01:33 PM #3 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 534 The video should be set to start in the right place, not at the beginning. I used "embed with time". Is it not doing that? Otherwise try 46:55 on the clock slider. My understanding of the potential function and how it comes from conservative forces was from this video at about 6:15 on the clock: That's what I assumed Susskind was saying but maybe I am wrong. One tends to switch off when one thinks one has heard it before somewhere else and perhaps miss the subtle differences and nuances in delivery. Last edited by kiwiheretic; Jul 29th 2017 at 02:59 PM.
Aug 2nd 2017, 01:29 PM   #4
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 Originally Posted by kiwiheretic The video should be set to start in the right place, not at the beginning. I used "embed with time". Is it not doing that? Otherwise try 46:55 on the clock slider. My understanding of the potential function and how it comes from conservative forces was from this video at about 6:15 on the clock: That's what I assumed Susskind was saying but maybe I am wrong. One tends to switch off when one thinks one has heard it before somewhere else and perhaps miss the subtle differences and nuances in delivery.
Why don't you just e-mail him and ask? Way to many people never resolve their problems understanding what an author writes/says because most people assume they won't get a response. I got a rather pleasant response when I e-mailed him. He seems quite pleasant in fact.

Aug 2nd 2017, 05:58 PM   #5
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 Originally Posted by Pmb Why don't you just e-mail him and ask? Way to many people never resolve their problems understanding what an author writes/says because most people assume they won't get a response. I got a rather pleasant response when I e-mailed him. He seems quite pleasant in fact.
Did you actually find his email address somewhere?

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