Second Order Differential Equation - Can't solve

In summary, the book's solution was x=a+Acos(wt+B). If you use the identity cos(a+b)=cos(a)cos(b)-sin(a)sin(b) on Acos(wt+B) you'll see that that form describes the same family of solutions.
  • #1
jumbogala
423
4

Homework Statement


The equation is
x'' - m2x - m2b = 0.

m and b are constants. x'' is the second derivative of x, with respect to time.

Homework Equations


The Attempt at a Solution


When I did a differential equations course, I was taught to find the characteristic equation of the differential equation, then solve that. I tried it, but obviously it gave me an answer that was e to the power of something.

Which is not in the right format. How did the book do this? Can anyone get me started?
 
Last edited:
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  • #2
x=a+Acos(wt + B) doesn't solve that equation. It does solve x''+ w^2*x-w^2*a=0. Is there a typo?
 
  • #3
Oh sorry, that negative should be a positive. The equation you gave is the correct one.
 
  • #4
jumbogala said:
Oh sorry, that negative should be a positive. The equation you gave is the correct one.

If it's positive then the real solutions to x''+w^2*x=0 aren't exponentials, are they?
 
  • #5
They aren't? (Sorry it's been a long time since I did differential equations. I can't remember methods to solve very well.)

Ok so I am remembering now that I have to find a solution to the homogeneous equation x'' + w^2*x = 0 first. So this has complex roots?

My old notes show that the solution should be in the form of e multiplied by a cos or sin still though.
 
  • #6
D^2+w^2=0 has complex roots. D=iw and -iw. So complex solutions are e^(iwt) and e^(-iwt). What kind of real solution do those produce? Is it coming back yet?
 
  • #7
You can use euler's identity: e^(iwt) = cos(wt) + isin(wt)

So I could try a general solution of that form. Like Acos(wt) + Bsin(wt) and use that.

The book's solution was x=a+Acos(wt + B), so if A and a are constants of integration this should work. I'm not sure where B comes from though, or if I have to include the i in front of the sin in my general solution.
 
  • #8
It is an inhomogeneous equation. It can be rewritten as:

[tex]
x'' - m^{2} \, x = m^{2} b
[/tex]

The corresponding homogeneous equation is:

[tex]
x'' - m^{2} \, x = 0
[/tex]

What is the general solution of this equation?

A particular solution of the inhomogeneous equation is a constant:

[tex]
x_{p}(t) = A
[/tex]

What value of A satisfies the equation?
 
  • #9
jumbogala said:
You can use euler's identity: e^(iwt) = cos(wt) + isin(wt)

So I could try a general solution of that form. Like Acos(wt) + Bsin(wt) and use that.

The book's solution was x=a+Acos(wt + B), so if A and a are constants of integration this should work. I'm not sure where B comes from though, or if I have to include the i in front of the sin in my general solution.

Yes, the general to the homogeneous solution is Acos(wt)+Bsin(wt). If you use the identity cos(a+b)=cos(a)cos(b)-sin(a)sin(b) on Acos(wt+B) you'll see that that form describes the same family of solutions.
 
  • #10
Dick said:
Yes, the general solution is Acos(wt)+Bsin(wt). If you use the identity cos(a+b)=cos(a)cos(b)-sin(a)sin(b) on Acos(wt+B) you'll see that that form describes the same family of solutions.

This is not true for the homogeneous equation posted in the op.

EDIT:

Never mind. :p
 

FAQ: Second Order Differential Equation - Can't solve

Why is it difficult to solve second order differential equations?

Second order differential equations involve two derivatives, making them more complex and challenging to solve compared to first order differential equations.

What are some common techniques used to solve second order differential equations?

Some common techniques include separation of variables, substitution, and the method of undetermined coefficients.

3. What should I do if I can't solve a second order differential equation?

If you are unable to find an analytical solution, you can try using numerical methods such as Euler's method or Runge-Kutta methods to approximate a solution.

4. Can technology assist in solving second order differential equations?

Yes, there are many software programs and online tools that can help you solve second order differential equations, such as Wolfram Alpha or MATLAB.

5. How can I improve my skills in solving second order differential equations?

Practice is key when it comes to solving differential equations. Work through different examples and use resources such as textbooks or online tutorials to improve your understanding of the concepts and techniques involved.

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