#1
Inertia: by Newton and by Einstein.
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a) Newton's first law of motion (inertia) is often stated as:
An object at rest stays at rest and an object in motion stays in motion
with the same speed and in the same direction unless acted upon
by an unbalanced force. (by an unbalanced outside force)

b) On September 28, 1905 Einstein wrote the paper:
 Does the inertia of a body depend upon its energy content?
As he realized the answer was:
 Yes, the inertia of the electrodynamics body depend upon its
energy content: E= Mc^2.

c) The electrodynamics body is photon and inertial movement of
photon depends on its own (not outside) energy content E=Mc^2.
Photon as a corpuscular at inertial constant speed has energy E=hc
Photon as a wave has energy E=h*f
How to explain the connection between these three formulas of photon?

d) Einstein showed that not only does a moving photon possess energy
but even a photon at rest contains rest energy E=Mc^2.
In 1927 Dirac "playing with beautiful equations" explained that every
quantum particle ( even a photon ) must have anti-brother (-E=Mc^2)
Question.
Which E=Mc^2 destroyed Hiroshima and Nagasaki: Einsteins or Diracs ?

e) Photon has hidden energy E=Mc^2
Electron has hidden energy E=Mc^2
Every quantum particle has hidden energy E=Mc^2
What is E=Mc^2? Nobody knows.
=.
In 1954 Albert Einstein wrote to his friend Michael Besso;
All these fifty years of conscious brooding have brought me
no nearer to the answer to the question, 'What are light quanta?'
Nowadays every Tom, Dick and Harry thinks he knows it,
but he is mistaken. (Albert Einstein, 1954)
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Best wishes.