Maths/astronomy help!

Feb 2009
8
0
For a star of constant density, calculate the fraction of the total mass inside a core of radius R/4, if the radius of the star is R.

I have no idea.

Thanks for any help.
 
Last edited:
Dec 2007
16,948
6,768
Zeitgeist
For a star of constant density, calculate the fraction of the radius R/4, if the radius of the star is R.

I have no idea.

Thanks for any help.
This question is incomplete. Please re-check it and then post the complete question.
 
Apr 2005
20,249
7,909
For a star of constant density, calculate the fraction of the radius R/4, if the radius of the star is R.

I have no idea.

Thanks for any help.
Sorry, but I have no idea what "calculate the fraction of the radius R/4" means. Since you mention density, do you mean calculate the mass out to radius R/4 or the fraction of the total mass that is? Do you know the formula for volume of a cube? Mass is just density times volume.
 
Feb 2009
8
0
Sorry, but I have no idea what "calculate the fraction of the radius R/4" means. Since you mention density, do you mean calculate the mass out to radius R/4 or the fraction of the total mass that is? Do you know the formula for volume of a cube? Mass is just density times volume.
Yes you are right. It should read For a star of constant density, calculate the fraction of the total mass inside the core of radius R/4, if the radius of the star is R.
 
Dec 2007
16,948
6,768
Zeitgeist
Yes you are right. It should read For a star of constant density, calculate the fraction of the total mass inside the core of radius R/4, if the radius of the star is R.
Start with the facts that mass = density times volume and that the volume of a sphere is \(\displaystyle \frac{4}{3} \pi r^3\).
 
Feb 2009
8
0
Start with the facts that mass = density times volume and that the volume of a sphere is \(\displaystyle \frac{4}{3} \pi r^3\).
So to find the mass of the core = (density x \(\displaystyle \frac{4}{3} \pi r^3\) ) / 4

Is that even right? And how could i get a fraction out of that without figures?
 
Dec 2007
16,948
6,768
Zeitgeist
So to find the mass of the core = (density x \(\displaystyle \frac{4}{3} \pi r^3\) ) / 4

Is that even right? And how could i get a fraction out of that without figures?
You have to substitute r = R/4 to get the mass of the core.

You have to substitute r = R to get the total mass of the star.

To get the required percentage, you have to calculate (core mass)/(total mass) times 100.