General solution of double integrals

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Discussion Overview

The discussion revolves around solving a second-order differential equation of the form \(\frac{d^2v}{dt} = \alpha\) using calculus, particularly through the use of double integrals. Participants explore the setup of these integrals and the correctness of the derived solution for \(v(t)\) within specified bounds.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant proposes a method to solve the differential equation using double integrals, suggesting the integral setup and providing a derived solution for \(v(t)\).
  • Another participant challenges the clarity of the left-hand side of the integral equation, questioning its validity and suggesting an alternative approach involving integration by parts.
  • A participant expresses a desire to verify the correctness of their procedure after deriving the solution.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the validity of the integral setup or the derived solution. There are competing views on the correctness of the approach and the interpretation of the integrals involved.

Contextual Notes

The discussion includes unresolved aspects regarding the assumptions made in setting up the double integrals and the interpretation of the bounds. There is also uncertainty about the implications of the derived solution in relation to the original differential equation.

d.arbitman
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I have a differential equation of the form and I want to solve it using calculus, as opposed to using a differential equation method.
\frac{d^2v}{dt} = \alpha

where v is a function of t i.e., v(t)
and \alpha is some constant.

How do I solve for v(t) if the time ranges from t_0 to t such that v'(t_0) and v(t_0) are the lower bounds of my double integrals?

Do I setup a double integral, such as the following?

\int_{v(t_0)}^{v(t)} \int_{v'(t_0)}^{v(t)} dvdv = \alpha\int_{t_0}^{t} \int_{t_0}^{t} dτ dτ

I get the following solution from evaluating that double integral:
v(t) = \frac{\alpha}{2}{(t-t_0})^2 +v'(t_0)(t-t_0) + v(t_0)
and I was wondering if the solution is correct?
 
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d.arbitman said:
I have a differential equation of the form and I want to solve it using calculus, as opposed to using a differential equation method.
\frac{d^2v}{dt} = \alpha

where v is a function of t i.e., v(t)
and \alpha is some constant.

How do I solve for v(t) if the time ranges from t_0 to t such that v'(t_0) and v(t_0) are the lower bounds of my double integrals?

Do I setup a double integral, such as the following?

\int_{v(t_0)}^{v(t)} \int_{v'(t_0)}^{v(t)} dvdv = \alpha\int_{t_0}^{t} \int_{t_0}^{t} dτ dτ

I get the following solution from evaluating that double integral:
v(t) = \frac{\alpha}{2}{(t-t_0})^2 +v'(t_0)(t-t_0) + v(t_0)
and I was wondering if the solution is correct?


Why wonder? Derive twice what you got and see if you get back your differential equation!

DonAntonio
 
I did, but I want to check if the procedure is correct.
 
d.arbitman said:
Do I setup a double integral, such as the following?
\int_{v(t_0)}^{v(t)} \int_{v'(t_0)}^{v(t)} dvdv = \alpha\int_{t_0}^{t} \int_{t_0}^{t} dτ dτ
The LHS makes no sense to me.
v''(u) = f(u)
\left[v'(u)\right]_{u=t_0}^w = \int_{u=t_0}^w f(u) du
v'(w) - v'(t_0) = \int_{u=t_0}^w f(u) du
\left[v(w)\right]_{w=t_0}^t - (t-t_0)v'(t_0) = \int_{w=t_0}^t \int_{u=t_0}^w f(u) du dw
v(t) = v(t_0) + (t-t_0)v'(t_0) + \int_{w=t_0}^t \int_{u=t_0}^w f(u) du dw
 

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