# Driven simple pendulum - system of first order ODEs

## Homework Statement

We have a driven pendulum described by the following differential equation:

$\frac{d^2\theta}{dt^2} = \frac{-g}{l}\sin(\theta) + C\cos(\theta)\sin(\Omega t)$

I need to turn this second order differential equation into a system of first order differential equations (then use a computer to solve the first orders, but that's not the problem here).

None needed

## The Attempt at a Solution

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We are told to use some numeric values: l = 10cm, g = 9.81m/s^2, capital omega = 5/s, C = 2/s^2, and we are told to turn the equation into a dimensionless equation using the following notation:

$\omega^2 = g/l$

$\beta = \frac{\Omega}{\omega}$

$\gamma = \frac{C}{\omega}$

$x= \omega t$

Now, putting these into the ODE gives

$\frac{d^2\theta}{dt^2} = \omega^2\sin(\theta) + \omega^2\gamma\cos(\theta)\sin(\beta x)$

But, the only way I can think of turning this into a system of first order ODEs is by using some dummy variable, y.

In other words, let

$\frac{d\theta}{dt} = y$

and

$\frac{dy}{dt} = \omega^2\sin(\theta) + \omega^2\gamma\cos(\theta)\sin(\beta x)$

Is there no way to get it all in terms of theta and x?

EDIT:
I accidentally posted this before it was complete because I hit the "enter" key. Is there a way to turn this feature off? I don't want to get into trouble over posting something which doesn't fit with the rules.

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## Answers and Replies

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Orodruin
Staff Emeritus
Homework Helper
Gold Member
What do you mean? You had it as a system including only theta and t and you rewrite it as a system of two first order equations. That necessarily has to involve another dependent variable, one you decided to call y. It is unclear why you introduce x unless you want a dimensionless number, but then you should replace all occurences of t with x and not mix the notation.

What do you mean? You had it as a system including only theta and t and you rewrite it as a system of two first order equations. That necessarily has to involve another dependent variable, one you decided to call y. It is unclear why you introduce x unless you want a dimensionless number, but then you should replace all occurences of t with x and not mix the notation.
Yeah, my thinking is the same and I think I just need to leave it at that (besides, numerically solving that system results in a graph which looks sensible).

But there is a reason why I need it in terms of x - a later part of the question is that it asks for a plot of theta against $\frac{d\theta}{dx}$, but the solution is for theta as a function of time. Also, it's a numerical solution - I don't actually know what theta is in analytical form so I can't just differentiate it with respect to x by hand.

My thinking is this:
I have the quantity x = omega * t. What If I just do $dx = \omega dt$, giving $\frac{1}{\omega} dx = dt$ and just substitute that into the system? I'm going to try that now.

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Orodruin
Staff Emeritus