I know I am late. But I think I understand what the person is asking. let me rephrase the question " Why does the earth attract everything to its center (or in one direction) when the earth is made up of different atoms each one pulling to its own center????"
- As others have already told, the gravity in simple terms adds up.
Let's assume earth to be made of uniform non-compressible substance(basically a sphere with non-compressible substance). So the density doesn't change. Now let's assume just a line of atoms from u on the surface to the center of the earth and for now, ignore the forces exerted by any other atoms outside this line.
Assume there are N atoms {a1,a2......aN} (including the center atom which is the 'aN' atom) each separated by distance from each other. So the total distance between u and the center atom is N*d. Now say f is the gravitational force between u and and first atom beside u i.e 'a1'. So the next atom 'a2' will be at 2d from u and its force on u would be 1/4 * f (inverse square law). So the total gravitational force F till a2 would be
= f+ (1/4)f. Similarly, the last one would have (1/N)^2 *f. Thus the total gravitational force on u from the center would be f +(1/4) * f + ..........+ (1/N)^2 * f < ((pi^2)/6) * f. There u have it for a line. Similarly when u consider every atom the resultant force is still toward the center.