Why Does Planetary Energy Increase in My ODE Simulation?

[jmtome2]

Homework Statement

OK so here goes.

I'm using an ODEsolver in java to plot the total energy over time of a planetary system. So I've been trying to calculate the rate of energy (per unit mass), \frac{E}{m}.

Homework Equations

The equation for total energy (per unit mass) of a planetary system is:
\frac{E}{m}=1/2\cdot v^2-\frac{G\cdot M}{r}

G is the gravitational constant
M is the mass of the sun (constant)
v is the velocity of the planet, v^2=v^{2}_{x}+v^{2}_{y}
r is the distance of the planet from the sun, r^2=x^{2}+y^{2}

Essentially I need help finding \frac{dE}{dt}

The Attempt at a Solution

The answer I got for the rate is:

\frac{dE}{dt}=v\cdot\left(a+\frac{G\cdot M}{r}\right)

where a is the acceleration of the planet, a^2=a^{2}_{x}+a^{2}_{y}

The problem is that everytime I throw this equation into the ODEsolver, I get a plot of ever-increasing energy as time goes on which I know is not correct.

Help anybody?

 

[kuruman]

Isn't dE/dt = 0 because energy is conserved? Am I missing something here?

 

[jmtome2]

YES! Of course it is... what was I thinking. But this creates a whole problem... I've got to figure out how to plot E now inside the program without using the ODEsolver, which hasn't been mentioned in the book yet. *sigh*

Thanks for clarifying :)