1 Answer

In modern physics you can view distances as seconds dealing with relationship to the speed of light.

To convert kilograms to joules you need to think in these terms, as in classical physics they measure different things-- kilograms measuring mass, and joules measuring energy.

To answer this question, consider Einstein's mass-energy equivalence formula E=mc^2.
In modern physics this would be represented with c, the speed of light to equal 1 with no units (as this is measuring seconds per second, as described above. This gives us the units of kg=kg, with energy on the left, and mass on the right.

In classical physics, E is measured in joules, and a joule is equal to one kg*(m^2/s^2). Going back to Einstein's equation of E=mc^2, we can now put c in SI terms (m/s).

We can round c to 3*10^8. Assuming our mass is one, which would give us a single joule on the right hand side of this equation, we see that E=1kg*(3*10^8)^2, or E(in joules)=9*10^16 kg*(m^2/s^2).

Therefore 1kg in essentially equal to 9*10^16 joules.  http://wiki.answers.com/Q/1_Kg_equals_how_many_joules