0
$\begingroup$

Assume a sun with mass $M$ and a planet with mass $m$. Assume at $t_0$ the planet is $r_0$ (distance) away from sun and has an initial velocity of $v_0$. Also, let’s assume the angle between the $r_0$ and $v_0$ is $\theta_0$ but I suppose if $\theta_0$ is $90$ degrees we can cover all the cases.

Are there any formulas and relations between $M, m, r_0, v_0$ that show wether the planet will make a circular, elliptical, parabolic or hyperbolic orbit ? (Or spiral orbit slowly moving towards the sun making smaller circles ) ?

$\endgroup$

3 Answers 3

4
$\begingroup$

Yes there is. Total energy is, in polar coordinates in the plane where the movement occurs:

$$E=\frac{1}{2}\,mv^2-G\frac{Mm}{r}=\frac{1}{2}\,m(\dot{r}^2+(r\dot{\theta})^2)-G\frac{Mm}{r}$$

A short rewrite using $C=\lVert\vec{L}\rVert/m$, the constant from Kepler's second law, yields:

$$E=\frac{1}{2}\,m\dot{r}^2+\underbrace{\frac{mC^2}{2r^2}-G\frac{Mm}{r}}_{=V_\text{eff}(r)}$$

The second and third terms form what's called the effective potential energy $V_\text{eff}$. Notice you necessarily have $V_\text{eff}(r)\leqslant E$, which can easily be interpreted on a graph:

Veff

In this example, $E<0$. Since $V_\text{eff}$ must remain below $E$, the only part of the plot that is physically accessible is $r\in[r_1,r_2]$. In other words, $r$ cannot go to infinity, it's a bound state. This is the elliptic case.

On the other hand, if $E>0$, $r$ cannot go to zero, but it can go to infinity. The movement isn't bounded, it's the hyperbolic case.

The special case $E=0$ is the parabolic one ($r$ can go to infinity, but then kinetic energy is zero).

The special case where $E$ is exactly at the bottom of the potential well is the circular movement (only one allowed value for $r$, here $r_0$).

In other words, the sign of $E$ will tell you exactly in which case you are. $E$ can be rewritten as:

$$E=\frac{1}{2}\,mv_0^2-G\frac{Mm}{r_0} \quad \begin{cases} >0 & \text{hyperbolic}\\ =0 & \text{parabolic}\\ <0 & \text{elliptic} \end{cases} $$

which should answer your question.

$\endgroup$
0
$\begingroup$

If a mass (like a planet) is orbiting a much larger mass (like the sun) under the action of gravity, it has kinetic energy (1/2)m$v^2$ and potential energy -GMm/r (negative because the zero is chosen to be at infinite r). If the total energy is negative, the orbit will be elliptical; zero, parabolic; and positive, hyperbolic. The path will be a spiral only if there is a mechanism which is causing a loss of energy. (The earth has been orbiting the sun for a very long time.)

$\endgroup$
0
$\begingroup$

For the planetary motion the total energy $$E=\frac{1}{2}mv^2-\frac{GMm}{r} \tag{1}$$ and the angular momentum $$L=mrv\sin\theta \tag{2}$$ will remain constant for all times $t$. Hence you can calculate them at time $t_0$ (with $r_0$, $v_0$, $\theta_0$).

From Newtonian mechanic the planetary orbit is known to be a conic section (never a spiral). Using the values for $E$ and $L$ from (1) and (2) you can calculate the eccentricity $\epsilon$ of the orbit. $$\epsilon=\sqrt{1+\frac{2EL^2}{G^2M^2m^3}}$$

From $\epsilon$ you can read off the shape of the orbit:

  • $\epsilon = 0$ : circular orbit
  • $0 < \epsilon < 1$ : elliptic orbit
  • $\epsilon = 1$ : parabolic orbit
  • $\epsilon > 1$ : hyperbolic orbit
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.