1. How did Einstein derive his most famous equation: $$E=mc^2$$

  2. Is the above equation a special case of $$E^2=m^2c^4+p^2c^2$$

  3. Its derivation?

  4. What is the difference between them?

  • $\begingroup$ do you mean Einstein's original derivation or any derivation will do? $\endgroup$ – Nikos M. Oct 29 '14 at 11:12
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    $\begingroup$ the original 1905 paper of Einstein on the electrodynamics of moving bodies already contains a form of this relation $\endgroup$ – Nikos M. Oct 29 '14 at 11:13
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    $\begingroup$ this is a second paper of Einstein on the relation between inertia and energy and contains a further form of the relation $\endgroup$ – Nikos M. Oct 29 '14 at 11:15
  • $\begingroup$ @Nikos M it would be helpful if you can furnish me with Einstein's original derivation, as long as it does not involve too much calculus(I am fine with basic calculus); although I will be indebted to you for any logically consistent proof $\endgroup$ – Quantum Sphinx Oct 29 '14 at 11:18
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    $\begingroup$ @Nikos M.: Note that age is not always decisive when deciding which is a duplicate of which. $\endgroup$ – Qmechanic Oct 29 '14 at 13:11