Hi I was wondering if anyone could help me with this equation. [tex]Fdx &= dm c^2 [/tex] First of all, excuse me for my limited knowledge of calculus, but how exactly can you just use the numerator of a derivative? What do Fdx and dm mean if they are not in respect to anything? Do they simply mean a change in x and a change in m? Secondly, I have seen this equation used to get to E=mc^2 through integration. How exactly do we integrate either side of this equation? What is the step by step process for doing this? Again, please excuse my limited math knowledge.