how much heat can be attributed to gravitational and frictional forces at work in the earths (or other planets) core?
Reply Heat is a form of energy. Frictional forces generate heat at the expense of motion, and the motion itself (creating the magnetic dynamo in the Earth’s core) is the result of heat-generated circulation, so it is probably a small factor (motions in the Earth’s core are exceedingly slow, anyway). Gravitational forces create heat only if the Earth shrinks (=everything is falling down to lower altitude). Stars can release a lot of energy that way, but I think the Earth is done with shrinking. Gravitational squeezing heats the interior of the satellite Io of Jupiter http://www.phy6.org/stargaze/Siomoons.htm but the effect here seems small.
