How Does x in Equation 6.24 Become a(1-cosӨ)?

Click For Summary

Homework Help Overview

The discussion revolves around understanding the substitution of x in the context of the Brachistochrone problem, specifically how it relates to the expression a(1-cosӨ) as seen in equation (6.24).

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants express confusion regarding the reasoning behind the substitution of x with a(1-cosӨ). Some suggest that it may be a clever substitution, while others question the theoretical basis for this choice. There are mentions of trying various approaches to find a suitable substitution.

Discussion Status

The discussion is active, with participants exploring different perspectives on the substitution. Some have offered insights into the nature of the substitution, while others are still seeking clarification on its theoretical justification. There is no explicit consensus yet, but various lines of reasoning are being examined.

Contextual Notes

Participants are grappling with the implications of the substitution and its effectiveness in simplifying the problem. There is mention of alternative substitutions and their potential usefulness, indicating a broader exploration of the topic.

Hamza Abbasi
Messages
47
Reaction score
4

Homework Statement


This is the solution of Brachistochrone .

upload_2015-9-26_19-47-36.png

upload_2015-9-26_19-48-17.png


Homework Equations

The Attempt at a Solution


I am very confused that how the x in equation(6.24) get its value a(1-cosӨ) ? What is the technique behind this solution of x?
 

Attachments

  • upload_2015-9-26_19-40-17.png
    upload_2015-9-26_19-40-17.png
    64.3 KB · Views: 811
  • upload_2015-9-26_19-44-33.png
    upload_2015-9-26_19-44-33.png
    55.1 KB · Views: 741
  • upload_2015-9-26_19-45-3.png
    upload_2015-9-26_19-45-3.png
    11.2 KB · Views: 652
Physics news on Phys.org
It is just a clever substitution that replaces x. How to find that:
- in theory: "oh, it is completely obvious that this substitution will lead to a nice solution!"
- in practice: "let's try several approaches until one of them works"
 
Why only a(1-cosӨ ? What is the theory behind this substitution?
 
There is no deeper theory. It is a clever definition of Ө. You can see that it works nicely later.

You could also choose x=Ө2 + 3 cos(Ө) but that wouldn't help.
 
Hamza Abbasi said:
Why only a(1-cosӨ ? What is the theory behind this substitution?
To motivate the substitution (6.24), first make the substitution x = au in (6.23). Then complete the square in the denominator. From your experience with integration, you should then see what might be another good substitution to further simplify the integral.
 

Similar threads

  • · Replies 7 ·
Replies
7
Views
3K
  • · Replies 5 ·
Replies
5
Views
4K
  • · Replies 5 ·
Replies
5
Views
2K
Replies
2
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
Replies
11
Views
3K
Replies
1
Views
2K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 4 ·
Replies
4
Views
3K
Replies
7
Views
2K