## Learning and learning

Dec 22, 2021 - 4 minute read - simulation classical mechanics

# Sim 3: Kepler Problems

We solve for the different types of orbits of the Kepler Problem (two body central force problems).

## Introduction

In the previous post we reduced our two body problem to a single effective potential problem:

$$U_{eff}(r) = U(r) + U_{cf}(r) = U(r) + \frac{l^2}{2 \mu r^2}$$

We can now determine the allowed types of orbits. To do this we are going to do some variable substitution, and determine $$r(\phi)$$ which will allow us to see the orbits more clearly. During this process we will define a new variable called eccentricity that determines the orbit.

## Solving for $$r(\phi)$$

We ended up last time with:

$$\mu \ddot{r} = F(r) + \frac{l^2}{\mu r^3}$$

For reasons I don’t quite understand yet, the problem becomes simpler if we replace $$r$$ with $$1/u$$ (something about quasilinear).

We also need to figure out how to write $$\ddot{r}$$ in terms of $$r(\phi)$$.

$$\ddot{r} = \frac{d}{dt} \frac{d}{dt} r = \frac{d \phi}{dt} \frac{d}{d\phi} \frac{d \phi}{dt} \frac{d}{d\phi} r = \dot{\phi}^2 \frac{d^2}{d\phi^2} \frac{1}{u} = (\frac{l u^2}{\mu }) \frac{d}{d \phi} (\frac{l u^2}{\mu}) (- \frac{1}{u^2}) \frac{d u}{d \phi} = - \frac{l^2 u^2}{\mu^2} \frac{d^2 u}{d \phi^2}$$

In step four we used the chain rule to partially evaluate $$\frac{d}{d \phi} \frac{1}{u} = - \frac{1}{u^2} \frac{d u}{d \phi}$$

So now we can substitute our result back into the original equation of motion:

$$u"(\phi) = -u(\phi) - \frac{\mu}{l^2 u(\phi)^2}F$$

Next we plug in our force equation: $$F = -G m_1 m_2 / r^2 = -\gamma u^2$$:

$$u"(\phi) = -u(\phi) - \frac{\mu}{l^2 u(\phi)^2} (- \gamma u^2) = -u(\phi) - \frac{\gamma \mu}{l^2}$$

This equation is surprisingly simple due to the fact that our force equation is an inverse squared force, the $$u(\phi)$$ terms cancel out. (Coincidentally I’m in the process of solving a related problem in a physics problem book, page 8 problem 8). I believe this implies that only inverse-square forces have stable orbits, but I haven’t proved it yet.

Since the last term is constant, the solution is just the constant plus a sinusoid:

$$u(\phi) = \frac{1}{r} = \frac{\gamma \mu}{l^2} + A \cos\phi = \frac{\gamma \mu}{l^2} (1 + \epsilon \cos\phi) = \frac{1}{c}(1 + \epsilon \cos \phi)$$

In the last stage we defined a new variable we will call the eccentricity (which will become obvious why soon), and $$c = \frac{l^2}{\gamma \mu}$$ for bookkeeping.

Solving for r:

$$r(\phi) = \frac{c}{1 + \epsilon \cos \phi}$$

## Types of orbits

### Bounded

Looking at our previous equation, if we assume $$\epsilon < 1$$, then r will oscillate between $$r_{min} = \frac{c}{1-\epsilon}$$ and $$r_{max} = \frac{c}{1+\epsilon}$$), which looks suspiciously like an ellipse (which is why we call $$\epsilon$$ the eccentricity). (You can prove it by converting to cartesian coordinates and using the identity $$\cos(\tan^{-1}(y/x)) == \frac{1}{\sqrt{1 + y^2/x^2}}$$, you will get the equation for an ellipse $$x^2/a^2 + y^2/b^2 = 1$$).

If $$\epsilon = 0$$ we will have a constant r, meaning a circle.

### Unbounded

We have two possible unbounded states, $$\epsilon = 1$$ and $$\epsilon > 1$$.

If $$\epsilon = 1$$, then r goes to infinity at $$\pi$$ and $$-\pi$$. We can show it’s a parabola by converting to cartesian coordinates:

$$r(\phi) = \frac{c}{1 + \cos\phi} = \sqrt{x^2 + y^2} = \frac{c}{1 + \frac{1}{\sqrt{1 + y^2/x^2}}}$$

Plugging into mathematica we get:

$$y^2 = c^2-2cx$$

Which is the equation of a (horizontal) parabola.

If $$\epsilon > 1$$ we will have a maximum $$\phi_{max}$$ which defines the range for some sort of hyperbola.

### Eccentricity Plot

There’s a fun plot here showing the trajectories of different eccentricities:

https://en.wikipedia.org/wiki/Orbital_eccentricity

## Simulation

Not a huge update, but now the simulation shows the eccentricity of the orbit.

## Conclusion

This is the last post on two body central force problems. I’m not sure what I’ll do next, but hopefully something just as fun and insightful.