Minimisation Problem (Euler-Lagrange equation)

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SUMMARY

The discussion centers on solving the Euler-Lagrange equation for a minimization problem where speed is proportional to distance, represented as v(x) = cx. The key equation used is d/dx ∂F/∂y' = ∂F/∂y. A participant, Plaetean, encounters difficulties in progressing beyond the initial steps, leading to results that equate to zero. Buzz advises that Plaetean overlooked the fact that y' is a function of x, suggesting that the derivative should include dy'/dx = y'' to properly apply the Euler-Lagrange equation.

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Plaetean
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Homework Statement


http://i.imgur.com/BV5gR8q.png

Homework Equations


d/dx ∂F/∂y'=∂F/∂y

The Attempt at a Solution


I have no problem with the first bit, but the second bit is where I get stuck. Since the question says the speed is proportional to distance, I have taken v(x)=cx where c is some constant of proportionality, then tried to proceed with solving the EL equation, however I end up with everything just being equal to 0 so I'm not sure how to proceed.

http://i.imgur.com/hDNPyG7.jpg
 
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Hi Plaetean:

I think you forgot that y' is a function of x.

Your (d/dx) ∂F/dy' does not include any dy'/dx = y''.

Hope this helps.
Regards,
Buzz
 
Plaetean said:

Homework Statement


http://i.imgur.com/BV5gR8q.png

Homework Equations


d/dx ∂F/∂y'=∂F/∂y

The Attempt at a Solution


I have no problem with the first bit, but the second bit is where I get stuck. Since the question says the speed is proportional to distance, I have taken v(x)=cx where c is some constant of proportionality, then tried to proceed with solving the EL equation, however I end up with everything just being equal to 0 so I'm not sure how to proceed.

http://i.imgur.com/hDNPyG7.jpg
What does your Euler-Lagrange equation boil down to if F is not a function of y?
 

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