Brachistochrone. Why a curve at all?

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    Brachistochrone Curve
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Discussion Overview

The discussion centers around the brachistochrone problem, specifically questioning why the solution involves a curve rather than a straight line when moving from a higher point to a lower point under gravity. Participants explore the implications of gravitational energy and the nature of optimal paths in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why a straight line would not be quicker than a cycloid, noting the counter-intuitive nature of the solution involving a curve.
  • Another participant suggests that the gravitational energy available to the body is crucial in understanding the path taken between points A and B.
  • A participant presents extreme cases where the quickest path is a straight line when the endpoint is directly below the start point, and discusses the implications of needing to go down and up again when the endpoint is at the same height as the start point.
  • One participant argues that minimizing path length alone is insufficient if average speed approaches zero, emphasizing the need to optimize both factors.
  • A later reply reiterates the initial question and critiques the reliance on intuition in scientific reasoning, suggesting that applying mathematical principles leads to the discovery of the cycloid as a faster solution.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the solution to the brachistochrone problem, with some emphasizing the role of gravitational energy and others focusing on the mathematical optimization of paths. The discussion remains unresolved regarding the intuitive understanding of the problem.

Contextual Notes

Participants highlight the limitations of intuitive reasoning in science and the importance of mathematical approaches, but do not resolve the underlying assumptions about the nature of optimal paths.

Gersty
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Why is the solution to the brachistochrone problem a curve at all? If the idea is to get from a higher point to a lower point under the influence of gravity alone, why is a straight line not quicker than a cycloid? It seems counter-intuitive that the shortest time would be along a curve and not a straight line. Even more so if that curve contains a segment where the object is moving upwards, and being slowed by gravity.
 
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Consider the extreme cases:

endpoint is exactly below startpoint : Here the quickest way actually is a straight line

endpoint at same height as startpoint : Here a straight line will need infinite time, because you will never start moving. The only way to reach the endpoint is to go down and up again.

So in general, you need a compromise, that will somehow get you going initially, but won't be too far away from the straight line.

In other words :

time = path_length / avg_speed

Minimizing path_length alone doesn't help if avg_speed goes to zero. You have to optimize both.
 
Last edited:
Gersty said:
Why is the solution to the brachistochrone problem a curve at all? If the idea is to get from a higher point to a lower point under the influence of gravity alone, why is a straight line not quicker than a cycloid? It seems counter-intuitive that the shortest time would be along a curve and not a straight line. Even more so if that curve contains a segment where the object is moving upwards, and being slowed by gravity.

This is a reasonable question - if you want Science to be 'reasonable' and 'acceptable' to your intuition. But why should you ever assume that Science works that way?
Throughout history, people have tried to approach things intuitively and it has led to many unsatisfactory results. Life became more predictable and things started to 'work' for us, once we started to apply the 'Scientific Approach'. Greek Philosophers sat down an pontificated about the way the Universe works and they didn't do Experiments. They got it wrong. Such a shame, because their maths was actually quite good.
The reason that the solution to the brachistochrone is not a single straight line path can be found by applying very straightforward maths to some very basic principles. The fact is that there is a Faster solution than a straight line - you could easily find a faster one by choosing two straight lines instead. So your intuition could then, possibly, accept that an even better solution could be obtained with an infinite set of short lines (a curve). It's a small step, then, to let the maths take over and give you a Cycloid curve.
 

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