The radius of the earth is 6.37 × 10⁶ m and its mass is 5.98 × 10²⁴ kg. 

(a) Find the average density of the earth. 

(b) The average density of the rocks at the earth’s surface is 2.7 × 10³ kg/m³. What must be true of the matter of which the earth’s interior is composed? Is it likely that the earth is hollow and peopled by another species, as the ancients believed?