# Question on general solution to harmonic EoM

1. Jan 21, 2017

### Vitani11

1. The problem statement, all variables and given/known data
An equation of motion for a pendulum:

(-g/L)sinΦ = Φ(double dot)

2. Relevant equations
L = length
g = gravity
ω = angular velocity
Φο = initial Φ
3. The attempt at a solution
The solution is Φ=Asinωt+Bcosωt

solving for A and B by setting Φ and Φ(dot) equal to zero respectively gives:

Φ= Φοcosωt

My question is how can you just go ahead and write down Φ=Asinωt+Bcosωt? My professor said it was because this was the only function that when differentiated twice give -sinωt but I can't just take that for what it is because I need to know how to arrive there.

2. Jan 21, 2017

### Ray Vickson

Your solution is incorrect: the differential equation $\ddot{\Phi} = -k \sin(\Phi)$ involves Elliptical functions. It is NOT of the form $\Phi = A \sin \omega t + B \cos \omega t$. See, eg.,
https://en.wikipedia.org/wiki/Pendulum_(mathematics) .

For very small angles $|\Phi| \ll 1$ we have $\sin(\Phi) \approx \Phi$, so for small angles the solution is of the form you want.

As for why that form applies: you can take several points of view.
(1) Two hundred years ago somebody discovered that form of solution, and we have been taught it ever since.
(2) You can recognize that the equation $\ddot{x} = k x$ has solution $x = e^{\sqrt{k} t}.$ When $k = -c^2$ that gives $e^{\pm i c t}$ and Euler's equation gives that as $\cos ct \pm i \sin ct.$ Taking the real and imaginary parts gives us the $\sin$ and $\cos$ forms.
(3) You can try to solve $\ddot{x} = -c^2 x$ as a power series $x = a_0 + a_1 t + a_2 t^2 + \cdots$ and use standard DE methods to determine the coefficients $a_0, a_1, a_2, \ldots$. You will find that you get a combination of the series expansions of $\sin ct$ and $\cos ct.$
(4) You can use the method of Laplace transforms.

There are other ways as well, but realisically, (1) is the easiest explanation. Your professor's explanation is 100% correct (except that both $\sin$ and $\cos$ will work).

Last edited: Jan 21, 2017
3. Jan 21, 2017

### Vitani11

Good explanation, thank you.

4. Jan 21, 2017

### Vitani11

When you say solve x(double dot) = -c2x as a power series in (3) are you referring to perturbation theory? .

5. Jan 21, 2017

### Ray Vickson

No, it has nothing to do with perturbation theory. Development of series solutions to DEs is one of the topics that gets covered in just about any DE course. Or, you can Google "series solution to differential equation".