On Sat, 12 Nov 2005 12:00:33 -0800, "Doink"
wrote:

OK, Energy = Mass X Speed of Light Sq.

You do have to use consistent units (j, kg, m/sec).

The reasons are the speed of light in a vacuum being constant for all
observers and the principle of equivalence. You can work it out using
a little algebra. See a textbook on modern physics for the details.