# E=mc^2 -The speed of light

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Why, when calculating values with this equation do we use the speed of light. And why do we use the value: 300000 and not the speed of light in meters per seconds or cms per second?

There may be an obvious answer, but I can find it.

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Your first question would be like asking, why is the equation E = mc^2 true. I certainly don't know.

As for the units, it's an arbitrary matter. If you want your energy in Joules, then use c= 3e8 m/s, or if you want them in microJoules etc. If you start using non-SI units you will not get Joules however. I don't have such a chart.

Analyzing the units:

mc^2 = kg m^2/s^2 = Joules

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Why, when calculating values with this equation do we use the speed of light. And why do we use the value: 300000 and not the speed of light in meters per seconds or cms per second?

You can use 1.8026175 x 1012 furlongs / fortnight for the speed of light if you want, and put in the mass in stones, and it will spit out the energy in terms of stone x furlong2/fortnight2. However, that's not a particularly useful relation.

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You can use 1.8026175 x 1012 furlongs / fortnight for the speed of light if you want, and put in the mass in stones, and it will spit out the energy in terms of stone x furlong2/fortnight2[/sup']. However, that's not a particularly useful relation.

Its so reassuring to know that I can calculate amounts of energy in terms of stones and furlongs. Thank you.

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You can use 1.8026175 x 1012 furlongs / fortnight for the speed of light if you want, and put in the mass in stones, and it will spit out the energy in terms of stone x furlong2/fortnight2[/sup']. However, that's not a particularly useful relation.

Stone is a unit of weight, not mass.

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Stone is a unit of weight, not mass.

I was having trouble finding an exotic Imperial unit of mass, heh

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E=mc2 is also just a unit thing. I usually use units where E=m.

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