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#26
Dec11-09, 07:07 AM
Mentor
P: 15,067
Quote Quote by baywax View Post
As far as I know gravity is a static force and represents only potential energy.
Changes in gravitational potential, however, manifest as kinetic energy; e.g., increased temperature. Gravitational potential energy led to the heating of the Earth in several ways.

1. Accretional heating. As the proto-Earth became ever larger, its gravity made incoming bolides hit ever harder. The proto-Earth's gravitational field gave those incoming bolides a boost in kinetic energy before they hit the proto-Earth in purely inelastic collisions. Energy is still conserved in such collisions. The kinetic energy of the incoming bolides was turned into heat.

2. Compressional heating. As proto-Earth grew in bulk it started to compress itself gravitationally. There is a considerable difference in self-induced gravitational potential energy between a planetesimal-sized loose collection of rocks and a smaller, but equally massive planetesimal-sized solid body. The resultant change in gravitational potential energy manifests itself in the form of heat.

3. Differentiation heating. The Earth is highly differentiated, with a dense iron core, a considerably less dense mantle, and an even less dense crust. This differentiation represents a *huge* change in gravitational potential energy, which once again manifests itself in the form of heat.