<a href="https:&#x2F;&#x2F;terrytao.wordpress.com&#x2F;2012&#x2F;10&#x2F;02&#x2F;einsteins-derivation-of-emc2-revisited&#x2F;" rel="nofollow">https:&#x2F;&#x2F;terrytao.wordpress.com&#x2F;2012&#x2F;10&#x2F;02&#x2F;einsteins-derivati...</a><p>It is one of the assumptions of Einstein, namely that there is a fixed max velocity c. To get Newton&#x27;s 1&#x2F;2mv^2 in the Taylor expansion requires a factor c^2.<p>Something else would lead to measurable deviations in slowly moving objects.