Kinetic energy becomes E = mc^2 when the velocity of a moving particle with mass m is equal to c (speed of light).

“…Kinetic energy becomes E = mc^2 when the velocity of a moving particle with mass m is equal to c (speed of light)...”

- yeah, that is, of course correct. However here is the essential nuance: in the Shevchenko-Tokarevsky’s informational physical model

https://www.researchgate.net/publication/354418793_The_Informational_Conception_and_the_Base_of_Physics it is rigorously , i.e. in complete accordance with all reliable experimental data, shown, that all/every particles always constantly move in the 4D space with metrics (cτ,X,Y,Z) of the Matter’s fundamentally absolute, fundamentally flat, and fundamentally “Cartesian”, (kinematical) [5]4D spacetime with metrics (cτ,X,Y,Z,ct), with 4D velocities that have identical absolute values be equal to the standard speed of light, c [“bold” means 4D vector].

That is determined by the main “kinematical” properties/parameters of the Matter’s ultimate base – the primary elementary logical structures –[5]4D binary reversible fundamental logical elements [FLE], which compose the [5]4D dense lattice, which is placed in the Matter’s 4D space above: FLE’s “size” , lP, and FLE’s “binary flip time interval”, tP; lP and tP are Planck length and Planck time, and so c=lP/tP,

- while every particle is specific cyclic disturbance in the FLE lattice, that is created if a lattice FLE is impacted by a 4D momentum P. Correspondingly every particle always moves in the lattice – and so, of course, in the 4D space – having the creating momentum P=mc, and energy E=Pc=mc2, m is inertial mass of the particle.

However in the 4D space the dimensions are essentially different – 3DXYZ dimensions differ from the cτ-dimension in that at not too high energy interactions the interactions happen – and are observable in everyday practice - in the 3D space, while what happens in cτ-dimension directly isn’t observable. So, say, “T-particles”, i.e. that have “rets masses”, are observable as “are at rest” [in 3D space], despite that move in cτ-dimension with the speed of light having momentums P0=m0c, E0=m0c2, m0 is “rest mass”; at that having rest masses particles and their antiparticles move in cτ-dimensions in opposite directions, what is also directly non-observable.

In the mainstream physics that isn’t known, and though in QED it is postulated that particles and antiparticles move “ahead and back in time”, this postulate really is only a mathematical trick that fits QED with experiment – since is adequate to the reality, in certain sense, though – in mainstream physics, including QED, Matter’s spacetime isn’t [5]4D one, but is 4D Minkowski spacetime.

Correspondingly in the mainstream the 3D space is some “true” space, and when a T-particle has momentum P>P0, it moves also in 3D space with 3D speed V, having “mainstream kinetic energy”, Ek=Pc-E0, which , by Pythagoras theorem, is Ek=m0c2/(1-V2/c2)1/2-m0c2;

at V

Sergey Shevchenko.

In kinetic energy, mass of a rigid body is always same. Only the massless body's velocity is equal to c (speed of light). In your case, what happened the mass? Preprint Mass-Energy Equivalence: Light Energy

Is mass a variable?

Chinnaraji Annamalai,

“…In kinetic energy, mass of a rigid body is always same. Only the massless body's velocity is equal to c (speed of light). In your case, what happened the mass? …Is mass a variable?…”

- to say about “mass” it is necessary before to understand – what does the physical notion and particle’s [body, etc.] parameter “mass” mean?. Really the answer to this question is given in the SS&VT informational physical model, the link see the SS post above: since the absolutely fundamental phenomenon/notion “Change” is logically self-inconsistent, every change of state of particle’s [body, etc.] , including creation of particle’s [body, etc.], is logically prohibited

- and so [practically] everything resists to changes, and this resistance is called “Inertia”. In physics Newton discovered that Inertia is universal in Matter, and has its measure, so introduced in physics the parameter “inertial mass”, m as “measure of inertia”. Besides he introduced other universal parameter of known for him bodies – “gravitational mass”, which fundamentally differs from inertial mass, and postulated, basing on yet Galileo experiments, that both masses are equivalent, so by using corresponding coefficient G it is possible in mechanics of objects in gravitationally coupled systems to use one “m”.

All/every particles, bodies, etc., in Matter have both – inertial and gravitational masses, at that yet well more 100 years ago it was experimentally discovered, that inertia of having “rest mass” particles changes at increasing of its speed in 3D space, [firstly along and orthogonal to particle’s velocity direction] and at that inertial mass m=γm0, [m0 is inertial “rest mass”, see the SS post above] , γ is the Lorentz factor; while at action of a force , F, on a moving with a 3D velocity V particle its “inertial mass”, if is defined relating to particle’s acceleration vector, depends also on angle between F and V .

At that the in mainstream physics and for you “massless” particles, [now directly observable only photons], though haven’t “rest mass”, nonetheless fundamentally obligatorily have inertial masses, and, if on a photon no forces act, it moves in the space along straight line and with constant speed – as that Newton postulated in 1-st mechanics law, having inertial mass, m=E/c2=ћω/c2. If a photon is impacted by some force with transmitting to it an additional momentum/energy, its frequency increases, and its inertial mass increases; not in γ times, but here that isn’t essential.

More see the SS post above and links in the post.

Cheers

Gravitational mass is determined by the strength of the gravitational force experienced by the body when in the gravitational field g. Inertia mass is determined in the empty space. These are under the General relativity theory.

E = mc^2 comes under the Special relativity theory.

Sergey Shevchenko, Your statement: "inertial mass m=γm0" is wrong.

Relativistic mass (m) = γm0 , which is true.

“…Sergey Shevchenko, Your statement: "inertial mass m=γm0" is wrong. Relativistic mass (m) = γm0 , which is true.….”

- both, “rest mass” and “relativistic mass”, are just “inertial masses”, just as that Newton defined in his 1-st “law of inertia” :

“Every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.”; see, say, https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion

Cheers

Please don't confuse me and others. And see the explanation of different masses below:

Rest mass is mass where the body is at rest.

Inertial mass is actual mass where the gravitational force or external force does not act on the body.

Relativistic mass is the product of rest mass and gamma(γ).

Sergey Shevchenko.

Please don't get confused with the Law of Inertia and Inertial Mass.

@Chinnaraji Annamalai

Dear Chinnaraji Annamalai,

Such a connection certainly exists. To see what kind of connection this is, you need to replace the parameter m in the formula you gave, which is not mass, but the relativistic inertia I, replace the parameter m with mo/sqrt(1 – v^2/c^2) and write this formula of yours in form E = mоc^2/sqrt(1 – v^2/c^2).

Now the resulting formula must be expanded into a Maclaurin series, using the fact that the expression 1/sqrt(1 – v^2/c^2) is expanded into a Maclaurin series, and the product of the parameters mоc^2 is present in this expansion as a constant factor.

You will get the expression mоc^2 (1+v^2/2c^2 + 3v^4/8c^4+5v^6/16c^6+35v^8/128c^8+….)

After the expansion, the mоc^2 factor will need to be put into brackets and you will get the expression mоc^2 + mоv^2/2 + mоc^2 (3v^4/8c^4+5v^6/16c^6+35v^8/128c ^8+….).

You will see that in the expression you received, mоc^2 is the rest energy of a moving body, mоv^2/2 is its classical kinetic energy, and the large term of the form

moc^2 (3v^4/8c^4+5v^6/16c^6+35v^8/128c^8+….) is a relativistic addition to the sum of rest energy and classical kinetic energy, that is, this addition is actually is the relativistic kinetic energy of body motion.

So for simplicity we can now write it all like this:

E = Eo + mоv^2/2 + Erel.kin (where rel.kin is the subscript, and Erel.kin is the relativistic addition to the classical kinetic energy.

The connection between these three energies is quite obvious.

I have written about this in detail on ResearchGate in the article “On the theory of inertia and the law of equivalence of relativistic inertia and energy”, section III “Relativistic total and kinetic energy”. Or everyone can read my monograph “Is the speed limit of light due to the growth of relativistic inertia? Isn’t Nature laughing at us?”

Link: https://strata.spb.ru/catalog/matematika/predel-skorosti-sveta-obuslovlen-rostom-relyativistskoy-inertsii-ne-smeetsya-li-nad-nami-priroda.html

There is an electronic version of the book. Good luck with your introduction.