# Homework Help: Tangetial acceleration in elliptical orbit

1. Jun 24, 2013

### bkraabel

1. The problem statement, all variables and given/known data
Is there any position in an elliptical orbit where the tangential component of the acceleration is greater than the component perpendicular to the tangential component? If so, what conditions on the orbit must there be for such a position to exist?

2. Relevant equations
For the tangential component of acceleration to be greater than the perpendicular component, the angle between $R$ and $v$ has to be greater than 45 degrees.
The semimajor axis is $2a$.
The distance from the Sun (focus) to the satellite is $R$
The mass of the Sun is $M$ and the mass of the satellite is $m$
The half-semimajor axis is related to the (constant) mechanical energy $E=K+U$ of the orbit as
$a=-\frac{GmM}{2E}$
The eccentricity of the ellipse is related to the (constant) angular momentum $L$ as
$e^2=1+\frac{2EL^2}{G^2m^3M^2}$
Combining the two previous expressions gives
$a^2(e^2-1)=\frac{L^2}{2Em}$

3. The attempt at a solution
From the geometry of an ellipse and the law of cosines, the best I can do is
$R(R+\frac{GMm}{2E})<\frac{L^2}{Em}$

But I can't seem to find a criterion that involves only constants of the motion. Any suggestions would be appreciated.

2. Jun 24, 2013

### tiny-tim

hi bkraabel!

you're making this too complicated

once you've decided that …
… this is no longer a physics problem, it's just a geometry problem

so forget energy, forget angular momentum:

what is the maximum angle that the line from the focus makes with the curve, for eccentricity e?