# Number density of earth

I was wondering what is the best way to calculate the number density of earth? Should we assume that the earth is mostly Iron and calculate the N from there or should we assume the average particle mass is the mass of a proton?

Also if a small black hole is created on earth and makes one passage through earth how can we calculate the mass it sweeps up? I calculated the mean free path of this black hole created from two protons colliding and I get a mean free path that is extremely large >>>>>> the diameter of earth. Does this mean the black hole will not interact with anything and go right through earth? I am not quite sure about this because don't black holes oscillate?

## Answers and Replies

DaveC426913
Gold Member
I was wondering what is the best way to calculate the number density of earth? Should we assume that the earth is mostly Iron and calculate the N from there or should we assume the average particle mass is the mass of a proton?
Or you could Wiki it: 5.5153 g/cm³

That is density not number density. Number density is number of particles over volume and basically my question is what is the best particle to use for the earth Iron? Proton? Electron?

Without looking at specific minerals, the best you could calculate would be a mean number density. Oxygen and silicon are the most abundant elements in the Earth's crust, so silicate (SiO_3) might give you a reasonable starting point for a rough calculation.