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Why is constant for the conversion of mass to energy square of the ligths speed? is it bedside it's the fastest real matter? .

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possible duplicate of Does $E$ really equal $mc^2$? – Qmechanic Nov 24 '11 at 19:33
Why? Why ask? This is the way it is. We get the best experimental predictions with this law, instead of any other one. That's why. – ja72 Nov 24 '11 at 22:29
Sorry, but this question doesn't make any sense... – J. C. Leitão Nov 25 '11 at 15:38

Because energy in joules, and joule is $\text{kg}\cdot\frac{\text{m}^2}{\text{s}^2}$ not $\text{kg}\cdot\frac{\text{m}}{\text{s}}$.

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Correct about always having the needed units, but also add that it fits all experimental data up to now. – anna v Nov 25 '11 at 4:54

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