Orbital Energetics Help

1. Feb 7, 2008

klarge

Given:
Planet is orbiting a star
G = 6.67259 x 10^-11 Nm^2/kg^2
Mstar = 1.99 x 10^30 kg
mplanet = 5.98 x 10^24 kg
distance from perihelion to star = 1.69 x 10^11 m
distance from aphelion to star = 1.745 x 10^11 m
potential energy at perihelion = -4.698 x 10^33 J

Need:
Kinetic energy at perihelion

To solve for perihelion kinetic energy I used GMm/2r and checked it with -.5 x potential energy, but apparently this was incorrect as my answer (2.3493 x 10^33 J) was incorrect.

Am I missing something? Or could the "correct" answer be incorrect (this has been known to occur).

2. Feb 7, 2008

D H

Staff Emeritus
The total energy is $$E=-\frac {GMm}{2a}$$, not $$-\frac {GMm}{2r}$$. The latter equation (the one you used) is only valid for circular orbits.