Solving differential equations using surface intersections

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Discussion Overview

The discussion revolves around the interpretation of surface intersections derived from a system of differential equations, specifically focusing on the equations dx/dt=f(x,y) and dy/dt=g(x,y). Participants explore the implications of these intersections in terms of equilibrium points and potential solutions to the differential equations, as well as their relevance in various physical contexts.

Discussion Character

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

Main Points Raised

  • Some participants propose that the intersections of the surfaces for f(x,y) and g(x,y) symbolize equilibrium points of the differential equations.
  • Others argue that these intersections represent points in x-y space where dx/dt=dy/dt, suggesting a relationship between the components of velocity.
  • A participant relates the concept to projectile motion, noting that at a 45-degree angle, the equality of velocity components implies equal initial velocities in both directions, but questions the broader significance for other angles.
  • Another point raised is that the intersections could indicate optimal points within the system, particularly in the context of growth rates.
  • One participant questions whether finding these surface intersections can aid in deriving solutions to the differential equations, referencing an article on "lie groups and surface intersections" as a potential resource.
  • A later reply suggests that while intersections may be useful for identifying specific optimal points, they do not provide much insight into the general solution of the differential equations unless f(x,y) equals g(x,y), which would be a boundary condition.

Areas of Agreement / Disagreement

Participants express differing views on the significance of surface intersections in relation to finding solutions to the differential equations. While some see potential value in identifying optimal points, others contend that these intersections do not contribute to understanding the general solution without specific conditions being met.

Contextual Notes

The discussion highlights the dependence on specific conditions and interpretations of the differential equations, as well as the potential limitations in applying the concept of surface intersections to general cases.

marellasunny
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Take for example a system of differential equations:
dx/dt=f(x,y)
dy/dt=g(x,y)

If I plot the surfaces for f(x,y) and g(x,y) and find their intersections,what does this symbolize in terms of the differential equations?
I'm thinking equilibrium points(??)
Can one go further and find the solutions to the differential equations from this?
 
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If I plot the surfaces for f(x,y) and g(x,y) and find their intersections,what does this symbolize in terms of the differential equations?
The intersection between the surfaces would be the set of points in x-y space which satisfy f(x,y)=g(x,y) ... which would mean that dx/dt=dy/dt=v(t) ... i.e. the x component of the velocity is equal to the y-component.
What does that suggest to you?
 
Simon Bridge said:
The intersection between the surfaces would be the set of points in x-y space which satisfy f(x,y)=g(x,y) ... which would mean that dx/dt=dy/dt=v(t) ... i.e. the x component of the velocity is equal to the y-component.
What does that suggest to you?

Well,in projectile motions where the throw angle is 45 deg,when the 2 velocity components are equal,this implies,the initial velocity in x-dir=inital velocity in y-dir.
But,for other throw angles,I do not know how knowing the point where the 2 velocity components are equal would help.

In terms of growth rate,it could also mean a optimal point of our system.

In turbine flow velocity diagrams,this means the absolute velocity=√2 *relative velocity between water and blades.

Q.But,again coming back to my question,can finding this surface intersections have any significance when it comes to finding the solution to the system of differential equations?A google search gives me a article on "lie groups and surface intersections".
 
You've pretty much answered your own question - the intersections would be useful in special problems like finding a particular kind of optimal point for the system. As a special trick for solving DEs ... it does not really tell you much about the general solution to the system of DEs unless you have reason to believe that f(x,y)=g(x,y). But that would be a boundary condition in the statement of the problem anyway wouldn't it?
 

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