I can't believe you people take everything so seriously. No one is saying Eienstein is wrong, I'm saying if the derivation technique I have seen in a physics text is exactly what what is used in original derivation (which I doubt 100%), then the factor '1' with mc<sup>2</sup> is not exact but approximation. <br /><br />Any one familiar with Binomial expansion is aware that the higher terms are usually dropped because they are 'negligible', but not 'zero'. <br /><br />In case of E=mc<sup>2</sup>, the binomial expansion is carried out on [1-(v/c)<sup>2</sup>]<sup>-1/2</sup>. As you can see object speed v is generally much much smaller than c. In such case higher terms in the expansion will have very very small effect and the factor can be approximated as '1'. But I don't know what the factor would be if v is comparable with c. <br /><br />That's why I asked if there's any method without approximation. This binomial expansion method may have been introduced in textbooks for its simplicity so that stupids like me can understand where E=mc<sup>2</sup> has come from. <br /><br /><br /><br />Ok, I get to think for a while and I'll correct myself. Rest energy is m<sub>o</sub>c<sup>2</sup>, with a factor of exactly '1'. But kinetic energy, when in motion, gets screwed up because of extra terms of binomial expansion, it is not <br /> m<sub>o</sub>v<sup>2</sup>/2 .<br /><br /> <div class="Discussion_UserSignature"> <font size="2" color="#ff0000"><strong>Earth is Boring</strong></font> </div>