[harrab]

طيب هذا كلام دكتور في يدرس فس امريكا شو رأيك فيه (من نفس الكتاب السابق concept of physics)

The Meaning of E = mc2
It has been said by many physicists and philosophers that the formula
E = mc2 means that “mass is equivalent to energy.” This is
philosophically false. It is not what Einstein said when he derived
this relation. What he said was that “the inertial mass of matter is a
measure of its energy content.”
In physics, as Newton originally postulated, the inertial mass of
matter is, by definition, a measure of its resistance to a change of
its state of rest or constant motion. The energy of matter, on the
other hand, is by definition, the capability of this matter to do work.
Thus, mass and energy are totally different concepts! What should
be said, instead of saying that mass is equivalent to energy, is that
mass (the inertia of matter) is a measure of the capability of this
matter to do work (its intrinsic energy). That is, analogously in
Hebrew, a tapooz (an orange) is not equivalent to an aitz (a tree)
because the orange comes from a tree! Similarly, the speed of a
body, v, is not equivalent to its kinetic energy, because the kinetic
energy depends on the speed v!