Comet with eccentricity of 0.7 and 200 year period

[joemama69]

Homework Statement

A comet of the star Sol has an eccentricity of .7. It takes 200 yrs to complete on orbit

1)find the average distance of the comet from Sol.

2) Find the perihelion and aphelion distances from Sol

3)find the velocity of the comet at perihelion

Homework Equations

The Attempt at a Solution

1) i know ecentricity = (F(2) - F(1))/c where F is the foci and c is the major axis how does this help me

 

[XanziBar]

You can use Kepler's 3rd law to find the average distance from the sun, knowing the period. By knowing the period you should also be able to find the semi-major axis of the orbit, and consequenctly the perihelion and Aphelion.

And finally the velocity at perihelion is related to the eccentricity of the orbit, and the distance of closest approach to the sun.

 

[joemama69]

but I am looking for the average distance of the comet from Sol, not the earth

...or am i reading it wrong

 

[XanziBar]

What does Kepler's 3rd law say?

 

[joemama69]

ohhhh i guess i should read more carefully

so r(commet)^3 / r(earth)^3 = T(commet)^2 / T(earth)^2

r(commet) = (200^2 * (2.28x10^11)^3)^1/3 = 7.797 x 10^12

 

[joemama69]

how do i find the perihelion and aphelion, our book doesn't even mention it but i believe they are the closest and farthers points of the orbit

 

[XanziBar]

You should double check your text (what book are you using, by the way?). But I think you can use the average distance as your semi major axis. Then you might find this helpful http://en.wikipedia.org/wiki/Apsis

Those equations are probably quoted to you somewhere.

 

[joemama69]

the book is 4th edition Physics for scientists & engineers by Giancoli