# How can I use Runge-Kutta4 to solve this orbit in polar coordinates?

I want to simulate a simple orbit of a planet moving around a star which is fixed in position. I have formulated the ODEs for this problem using Lagrangian Mechanics and have found the equations of motion for a 2D orbit in polar coordinates.

I have found these to be:

$$\ddot r - r \dot \theta^2 = \frac{-GM}{r^2}$$

and:

$$r\ddot\theta+2\dot r\dot \theta = 0.$$

I would like some help formulating the problem in a way in which I can solve for $$(r,\theta)$$ using the RK4 method (I am aware this method is non-symplectic but I want to use this method first and will probably update to use velocity vertlet or something later on)

I have never used RK4 before to solve for two variables simultaneously and this is where my confusion in how to proceed arises.

• I'm voting to close this question as off-topic because it's a request for codes rather than physics – Kyle Kanos Jul 28 at 18:40
• @KyleKanos I disagree. The OP didn't ask for code. He only needed the concept how to transform second-order ODEs to a first-order ODE, as required for Runge-Kutta. – Thomas Fritsch Jul 28 at 18:49
• Solving the two-body problem using cartesian coordinates in an inertial frame and without using relative coodinates is by far the most instructive way to do it. My students learn a lot of mechanics doing that way. Velocity Verlet algoritm is recommended, not oly because it is symplectic, which is important for a problem where angular momentum should be conserved, but also because it is very simple to implement and very efficient (one evaluation of force per step). – GiorgioP Jul 29 at 6:42
• @ThomasFritsch then he should have opened any book on numerical methods where this surely is discussed. – Kyle Kanos Jul 29 at 11:26

To use numerical simulation program , you have to transformed the differential equations to first order differential equations (1)

$$\vec{\dot{y}}=\vec{f}(\vec{y},t)\tag 1$$

Example:

$$\ddot{r}-r\,\dot{\theta}=-\frac{GM}{r^2}\tag 2$$

$$r\ddot{\theta}+2\,\dot{r}\dot{\theta}=0\tag 3$$

new variables :

$$y_1=\dot{r}$$

$$y_2={r}$$

$$y_3=\dot{\theta}$$

$$y_4={\theta}$$

equation (2):

$$\dot{y}_1-y_2\,y_3=-\frac{G M}{y_2^2}$$

equation (3):

$$\dot{y}_3\,y_2+2\,y_1\,y_3=0$$

thus:

$$\vec{\dot{y}}=\vec{f}(\vec{y})$$

$$\begin{bmatrix} \dot{y}_1 \\ \dot{y}_2\\ \dot{y}_3 \\ \dot{y}_4\\ \end{bmatrix}=\begin{bmatrix} -\frac{G M}{y_2^2}+y_2\,y_3 \\ y_1\\ -\frac{2\,y_1\,y_3}{y_2}\\ y_3 \end{bmatrix}$$