Which energy source contributes the most to heating the ground?
a. friction
b. solar radiation
c. radioactive decay
d. the differential rotation of earth's core

Solar Radiation, aka sunlight. The sun is the primary source of heat for the planet. Without it, we would freeze at the surface. Friction is negligible since we don't create near enough, if there was radioactive decay heating up the surface more than the sun, we would be dead, and the last one is just a teaser.

An  atom  (ours, not carroll's) consists of a positively charged  nucleus, surrounded by one or more negatively charged particles called  electrons. the positive charges equal the negative charges, so the atom has no overall charge; it is electrically neutral. most of an atom's mass is in its nucleus; the mass of an electron is only 1/1836 the mass of the lightest nucleus, that of hydrogen. although the nucleus is heavy, it is quite small compared with the overall size of an atom. the radius of a typical atom is around 0.1 to 0.25 nanometres (nm), whereas the radius of a nucleus is about 10-6  nm.  *  if an atom were enlarged to the size of the earth, its nucleus would be only 200 feet in diameter and could easily rest inside a small football stadium.read more on -