Solve ODE: Break into System of ODEs

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In summary, the conversation discusses how to break an equation into a system of ODEs. One suggestion is to use multiple variables and equations, but there is still a lack of information to fully solve the problem. Another potential solution is to use additional information about the variables, such as positive and constant acceleration and positive velocity, but it is uncertain if this is enough to fully solve the system.
  • #1
Dustinsfl
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Is there a way to break this up into a system of ODEs?
$$
L\ddot{\theta} + \dot{x}\dot{\theta} + \ddot{x}\theta = 0
$$
 
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  • #2
It's a little unusual that you have a single second-order ODE in two independent variables, but you could just do this:
\begin{align*}
x_{1}&=x \\
x_{2}&= \dot{x} \\
y_{1}&= \theta \\
y_{2}&= \dot{ \theta} \\
0&=L \dot{y}_{2}+x_{2} y_{2}+ \dot{x}_{2} y_{1}.
\end{align*}

There is no $x$ in the original ODE, so you could theoretically integrate that once immediately by essentially leaving out the first equation.
 
  • #3
dwsmith said:
Is there a way to break this up into a system of ODEs?
$$
L\ddot{\theta} + \dot{x}\dot{\theta} + \ddot{x}\theta = 0
$$
Notice that $\dot{x}\dot{\theta} + \ddot{x}\theta = \frac d{dt}(\dot{x}\theta)$, so (assuming that $L$ is a constant) the equation can be written $\frac d{dt}(L\dot{\theta} +\dot{x}\theta) = 0$. You can integrate this once, to get $L\dot{\theta} +\dot{x}\theta = $ const. But you still have the situation of two dependent variables and only one equation, so I don't see how you can go beyond there without further information.
 
  • #4
Opalg said:
Notice that $\dot{x}\dot{\theta} + \ddot{x}\theta = \frac d{dt}(\dot{x}\theta)$, so (assuming that $L$ is a constant) the equation can be written $\frac d{dt}(L\dot{\theta} +\dot{x}\theta) = 0$. You can integrate this once, to get $L\dot{\theta} +\dot{x}\theta = $ const. But you still have the situation of two dependent variables and only one equation, so I don't see how you can go beyond there without further information.

I know that acceleration is positive and constant and velocity is positive. Does that offer enough information?
 
  • #5


Yes, it is possible to break this ODE into a system of ODEs. We can rewrite the equation as:
$$
\begin{cases}
\dot{\theta} = \phi \\
\dot{\phi} = -\frac{\dot{x}\phi + \ddot{x}\theta}{L}
\end{cases}
$$
This system of ODEs can then be solved using numerical methods or analytical techniques. Breaking the original ODE into a system of ODEs can often make it easier to solve and can also provide more insight into the behavior of the system.
 

FAQ: Solve ODE: Break into System of ODEs

What is an ODE?

An ODE, or ordinary differential equation, is a mathematical equation that describes the relationship between a function and its derivatives. It is commonly used in physics, engineering, and other scientific fields to model a wide range of phenomena.

How do you solve an ODE?

An ODE can be solved by finding the function that satisfies the equation. This can be done analytically, using mathematical methods such as separation of variables or the method of integrating factors. It can also be solved numerically, using techniques such as Euler's method or Runge-Kutta methods.

What is a system of ODEs?

A system of ODEs refers to a set of two or more ODEs that are connected by one or more independent variables. This means that the derivatives of the functions in the system are dependent on each other. Solving a system of ODEs involves finding a set of functions that satisfy all of the equations simultaneously.

Why is it important to break an ODE into a system of ODEs?

Breaking an ODE into a system of ODEs can make it easier to solve, as each equation can be treated separately. This can also provide more insight into the behavior of the system and can help to identify relationships between the different functions in the system.

What are some real-world applications of solving a system of ODEs?

Solving a system of ODEs is used in many fields, including physics, chemistry, biology, economics, and engineering. It can be used to model complex systems such as population dynamics, chemical reactions, and electrical circuits. It is also used in the design and analysis of control systems and in predicting the behavior of physical systems.

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