Are these the correct expressions for ## dF/dy' ##?

In summary, the expressions for ## dF/dy' ## are: a) ## \frac{1}{4}(1+y'^2)^{\frac{-3}{4}}\cdot 2y' ## b) ## cos (y') ## c) ## e^{y'} ##
  • #1
Math100
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Homework Statement
Find the expressions for ## dF/dy' ## when
a) ## F(y')=(1+y'^2)^{\frac{1}{4}} ##
b) ## F(y')=sin (y') ##
c) ## F(y')=exp(y') ##
Relevant Equations
None.
a) ## dF/dy'=\frac{1}{4}(1+y'^2)^{\frac{-3}{4}}\cdot 2y' ##
b) ## dF/dy'=cos (y') ##

I just took the derivatives above and found out these expressions, but may anyone please check/verify to see if these expressions for ## dF/dy' ## are correct? Also, I do not understand part c). What does 'exp' indicate in here?
 
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  • #2
Math100 said:
Homework Statement: Find the expressions for ## dF/dy' ## when
a) ## F(y')=(1+y'^2)^{\frac{1}{4}} ##
b) ## F(y')=sin (y') ##
c) ## F(y')=exp(y') ##
Relevant Equations: None.

a) ## dF/dy'=\frac{1}{4}(1+y'^2)^{\frac{-3}{4}}\cdot 2y' ##
b) ## dF/dy'=cos (y') ##

I just took the derivatives above and found out these expressions, but may anyone please check/verify to see if these expressions for ## dF/dy' ## are correct? Also, I do not understand part c). What does 'exp' indicate in here?
##\exp(y')## means ##e^{y'}.##

What you wrote is correct, but why is there a prime at the ##y##'s? The same could be written with just ##y## or ##t## as the variable name. ##y'## normally indicates a derivative.
 
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  • #3
fresh_42 said:
##\exp(y')## means ##e^{y'}.##

What you wrote is correct, but why is there a prime at the ##y##'s? The same could be written with just ##y## or ##t## as the variable name. ##y'## normally indicates a derivative.
I don't know either. So what should the book normally express these primes then, instead? Also, if ## exp(y') ## mean ## e^{y'} ##. Then the expression for ## dF/dy' ## is ## dF/dy'=e^{y'} ##?
 
  • #4
Math100 said:
I don't know either. So what should the book normally express these primes then, instead? Also, if ## exp(y') ## mean ## e^{y'} ##. Then the expression for ## dF/dy' ## is ## dF/dy'=e^{y'} ##?
Yes.
 
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  • #5
fresh_42 said:
Yes.
Thank you so much for verifying!
 

FAQ: Are these the correct expressions for ## dF/dy' ##?

What does ## dF/dy' ## represent in the context of calculus of variations?

In the context of the calculus of variations, ## dF/dy' ## represents the partial derivative of a functional ## F ## with respect to the derivative of a function ## y ##. This is often encountered in problems where one seeks to find the function ## y(x) ## that makes a given functional stationary.

How do you compute the partial derivative ## dF/dy' ##?

To compute the partial derivative ## dF/dy' ##, identify the functional ## F ## and treat ## y' ## as an independent variable. Differentiate ## F ## with respect to ## y' ## while keeping other variables constant. This involves applying standard differentiation rules to the expression involving ## y' ##.

What is the significance of ## dF/dy' ## in the Euler-Lagrange equation?

The term ## dF/dy' ## is crucial in the Euler-Lagrange equation, which is used to find the function that makes a functional stationary. The Euler-Lagrange equation is given by ## \frac{d}{dx} \left( \frac{\partial F}{\partial y'} \right) - \frac{\partial F}{\partial y} = 0 ##. Here, ## \frac{\partial F}{\partial y'} ## is the partial derivative of the functional with respect to ## y' ##.

Can ## dF/dy' ## be zero, and what does that imply?

Yes, ## dF/dy' ## can be zero. If ## dF/dy' = 0 ##, it implies that the functional ## F ## does not depend on the derivative ## y' ##, or that the dependence on ## y' ## is such that its contribution to the derivative is null. This can simplify the problem significantly, as the Euler-Lagrange equation may reduce to a simpler form.

How does one verify if the expression for ## dF/dy' ## is correct?

To verify if the expression for ## dF/dy' ## is correct, re-evaluate the partial derivative step-by-step, ensuring that all differentiation rules are applied correctly. Additionally, check the consistency of the units and dimensions if applicable. Comparing with known results or using symbolic computation tools can also help confirm the correctness.

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