How does one get the solution to the differential equation for SHM?

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Discussion Overview

The discussion revolves around the derivation and understanding of the solution to the differential equation for simple harmonic motion (SHM). Participants explore the mathematical foundations and reasoning behind the solution, as well as the methods used to arrive at it.

Discussion Character

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

Main Points Raised

  • One participant outlines the derivation of the SHM equation and questions how the solution x = A cos (wt + ∅) is obtained.
  • Another participant suggests that the solution is based on the exponential function ei k θ, noting that e^x is its own derivative and relating it to Euler's equation.
  • A different participant references a previous discussion on the same topic, providing a link to their answer.
  • One participant proposes that guessing the solution in the form of exponential functions is a valid approach, leading to the discovery of complex solutions that can be rewritten as sine and cosine functions using Euler's equation.
  • Another participant emphasizes that guessing and checking is an effective method for finding solutions to differential equations, while also noting the limitations of computational tools in the guessing process.
  • A later reply suggests that there are systematic methods involving eigenvectors that could lead to the solution, but acknowledges that these methods may be too complex for beginners.

Areas of Agreement / Disagreement

Participants express varying opinions on the methods for deriving the solution, with some advocating for guessing and checking while others suggest more systematic approaches. There is no consensus on the best method to arrive at the solution.

Contextual Notes

Some participants mention the complexity of the explanations in textbooks and the potential for confusion among learners, indicating that the discussion may involve differing levels of familiarity with the underlying mathematics.

Who May Find This Useful

Students and individuals interested in understanding the mathematical foundations of simple harmonic motion and differential equations may find this discussion relevant.

mahrap
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I understand the derivation for the simple harmonic motion equation:

F = -kx ( in a 1-D case)

acceleration = x''(t) = (-k/m)x

so x''(t) + (k/m)x = 0

But why is the solution to this equation

x = A cos (wt + ∅ )

How does one come up with this solution? I tried understanding this by reading my textbook however I get very confused. Any help is appreciated. Thank you.
 
Last edited:
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I think initially

ei k θ

Is the real basis of the solution.

That's because e^x is its own derivative.

Euler's equation relates e^i theta to sin and cos functions hence the solution you see.
 
You basically guess that the solution is an exponential because of the form of the DE. So something along the lines of A_e^-wx+B_e^wx, and then you discover that w is complex and you rewrite those complex exponentials into sines and cosines with eulers equation.
 
mahrap said:
How does one come up with this solution?
Guess and check. Unfortunately, that is one of the most effective ways of coming up with solutions to differential equations. Computers can be helpful with that, they aren't as good at the guessing part, but they can do the checking part very quickly.
 
@mahrap: You don't have to guess. If you enjoy eigenvectors and all that stuff, then it follows naturally. But if you are learning this the first time, then maybe it is too long a detour. That is probably why your textbook is giving a weird explanation. They want to reassure you that there is a proper way to get the answer, but it would take up too much writing to actually explain it.
 

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