Calculating radius of the core with just densities

1. Sep 20, 2011

bertwan

1. The problem statement, all variables and given/known data
Calculate the radius of the Earth's core using the following data:

Pe = 5,515 kg/m^3 (earth's average density)
Pc = 11,000 kg/m^3 (core's density)
Pm = 4,450 kg/m^3 (mantle-crust density)

Re = 6377km (radius of the earth)

2. Relevant equations
im guessing we should be using the gravitational equation

g = (GM)/r^2 for different shells (might be wrong)

g= gravitational acceleration
G= Gravitational constant
M = mass of sphere with radius of "r"

3. The attempt at a solution

i tried the above equation but got no where with this. i need help!!!

2. Sep 20, 2011

Staff: Mentor

You're dealing with densities, not forces. If the Earth consists of an inner sphere of one density and an outer shell of another density, how would you calculate the average density? Suppose that the inner sphere has radius rc and that the outer shell extends to radius Re (radius of the Earth).