How did Einstein get to this equation?...I mean the math behind it and why E=mc2? why not F=mc2? we can get so much force from m*c2 right? but why did Einstein write E=mc2?
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Here is Einstein's original paper on the topic, which is actually pretty readable: http://fourmilab.ch/etexts/einstein/E_mc2/www/
Of course there's no particular reason that we have to go about things exactly the same way as Einstein. There are various ways of getting this result.
In general, you're not well advised to try to learn relativity from scratch simply by posting questions on this forum. You need a book to present things in an organized way. Some good books about special relativity are (from easiest to hardest):
Takeuchi, An Illustrated Guide to Relativity
Mermin, It's About Time: Understanding Einstein's Relativity
Taylor and Wheeler, Spacetime Physics
The advantage of Takeuchi and Mermin is that they require very little background in math or physics. Their disadvantage is a total lack of connection to experiment.
By the way, F = mc^2 makes no sense. The right hand side is a unit of energy, the left is a unit of force. In fact, once you read up on it a little, you'll see that E = mc^2 applies to an object just sitting there - it has an inherent energy associated with it just by existing. Talking about forces in that context is like asking how much speed I can get out of a car that has no wheels.How did Einstein get to this equation?...I mean the math behind it and why E=mc2? why not F=mc2? we can get so much force from m*c2 right? but why did Einstein write E=mc2?