Trouble solving this differential equation

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Discussion Overview

The discussion revolves around solving a differential equation with given initial conditions. Participants explore the relationship between the amplitude "A" and the solution "y(t)" of the equation, as well as the implications of the initial conditions on the uniqueness of "A".

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses difficulty in solving the differential equation and seeks assistance in determining the amplitude "A".
  • Another participant argues that "A" is not uniquely determined by the initial conditions provided, suggesting that varying "A" would still yield a valid solution for "y(t)".
  • A participant confirms the need to find "y(t)" before determining "A", indicating a misunderstanding of the relationship between the two.
  • Further clarification is provided that "A" can take on multiple values, and the solution for "y(t)" can be expressed in terms of "A" and "t".
  • One participant introduces the concept of implicit differentiation but expresses uncertainty about its correctness in this context.
  • Another participant challenges the use of implicit differentiation, stating that the derivatives "y'" and "y''" are already defined with respect to time, and further differentiation does not provide additional insights.
  • It is reiterated that the initial value problem has a unique solution for every possible value of "A", emphasizing that "A" cannot be determined from the given information.

Areas of Agreement / Disagreement

Participants generally agree that "A" is not uniquely determined by the initial conditions, but there is disagreement on the necessity of finding "y(t)" before determining "A" and the appropriateness of the differentiation approach suggested.

Contextual Notes

The discussion highlights limitations in the initial conditions and the implications for determining "A". The relationship between the variables and the methods of solving the differential equation remain unresolved.

BViper
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im having a lot of trouble solving this differential equation. can someone please help.

initial conditions: y(0)=0.1
y'(0)=0
30000*y''+1462163*y+100000*y'=A*sin(6.98t)

im trying to get A (the amplitude). thanks for the help
 
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"A" is not uniquely determined by the conditions that you gave. Varying "A" would change the solution to the equation, but the solution would still exist.

Are you looking how to solve y(t)?

cookiemonster
 
yeah that's it.. got to get y(t) before i can get A
 
No, you don't need to get y(t) before you get A. As I said before, A is not uniquely determined in this problem. The problem does not require that A take a single value or even a finite number of values. A could be anything (okay, almost anything) and y(t) would still exist.

If you'd like to solve for y(t) in terms of A and t, then that can be done. If you'd like to do that, then perhaps you could supply more information regarding what method you want see (perhaps one you've been using recently?).

cookiemonster
 
implicit differentiation...


initial conditions:
y(0)=0.1
y'(0)=0
30000*y''+1462163*y+100000*y'=A*sin(6.98t)
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Uncertain if this approach is correct...

Implicit Differentiation:
ay" + by + cy' = A*sin(d*t)

(d/dt)[ay" + by + cy'] = (d/dt)[A*sin(d*t)]

(d/dt)[ay"] + (d/dt)[by] + (d/dt)[cy'] = (d/dt)[A*sin(d*t)]

a(dy"/dt) + b(dy/dt) + c(dy'/dt) = A(d/dt)[sin(d*t)]

a(dy"/dt) + b(dy/dt) + c(dy'/dt) = A*cos(d*t)

 
Last edited:
Orion1: The only variables mentioned in this problem are y and t.
y' and y" already ARE the derivatives with respect to time. There is nothing gained by differentiating again.

Cookiemonster's point was correct: the initial value problem:
initial conditions:
y(0)=0.1
y'(0)=0
30000*y''+1462163*y+100000*y'=A*sin(6.98t)

Has a unique solution for every possible value of A. It is impossible to "determine A" from what is given.
 

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