This is the first part of the Explanation. It is indeed E²=m²+p², but the p is the _relativistic_ impulse. For practical proposes you usually want the classical

This is the first part of the Explanation. It is indeed E²=m²+p², but the p is the _relativistic_ impulse. For practical proposes you usually want the classical impulse p_0 in this equitation. And is a way easier to see how c shows up in the conversion between these two.