Consider two iron spheres, each of diameter 100mm, which are just touching. At what distance r from the center of the earth will the force of mutual attraction between the contacting spheres be equal to the force exerted by the earth on one of the spheres? Here's my work... d = distance between center of spheres r = distance between center of earth to the center of sphere me = mass of earth m = mass of sphere=m1=m2 F = G(m^2)/(d^2) = I get some number with (m^2)...so 1 unknown. then I make this equal to... F = G*me*m/(r^2) = I get some number with m/(r^2)...so 2 unknowns. I am confused as to what I am suppose to do beyond this... Problem seems to be asking me to set 2 equations to equal to each other and solve for r but it won't work...what am I doing wrong? Thank you.