Work done in elliptical orbits

[CaptainADHD]

Homework Statement

http://img47.imageshack.us/img47/7206/wtffffftv0.th.jpg http://g.imageshack.us/thpix.php

Homework Equations

Work is the dot product of force vector and displacement vector. Centripetal acceleration is velocity squared divided by radius -- multiply by mass to get force.

The Attempt at a Solution

Parallel does no work, perpendicular speeds it up ----->>> which is wrong (failed this question).

I understand that objects in circular orbit have zero acceleration and thus constant speed. But with elliptical orbits, the body is speeding up or slowing down depending on its position relative to the body exerting gravitational force upon it.

What exactly is parallel to the planet itself? I'm having a hard time understanding how to apply force components when I don't know what the angles are.

 

[krausr79]

If I'm following this correctly, it will be the reverse answer of what you gave. Work is done only if there is some motion in the same direction of the force. By parallel, they mean parallel to the movement of the planet, or in the same direction as the V arrow. Perpendicular is perpendicular to the motion, or the force that tries to smash the planet into the sun. It doesn't move that way, so no work by perpendicular, and parallel speeds it up.

 

[CaptainADHD]

If I'm following this correctly, it will be the reverse answer of what you gave. Work is done only if there is some motion in the same direction of the force. By parallel, they mean parallel to the movement of the planet, or in the same direction as the V arrow. Perpendicular is perpendicular to the motion, or the force that tries to smash the planet into the sun. It doesn't move that way, so no work by perpendicular, and parallel speeds it up.

Thanks