The conclusion drawn by Einstein in his study of special relativity was that a body gaining kinetic energy increases its mass. Thus heating a body increases its mass a little. Radiation emitted by the sun contains energy and by strength of this has mass. The term for the relation between mass and total energy is Einstein's mass energy formula:

* E = mc^^2*

It follows that the total energy of a body is proportional to its mass. The massess of all bodies increase with the increase in their energies. In general any variation in the energy of a system(body, particle etc.,) in any form entails a proportional variation in mass.

I don't enjoy having always to correct you, but I have to do it, in cases like this one, sorry!

1. A body gaining kinetic energy do really increases its mass *but only* if the body's center of mass remains stationary (so it can only have rotational kinetic energy).

The same for any kind of body (even massless): it increases mass, when it acquires energy, only if that body's total momentum is and remain zero:

E

^{2} = (cp)

^{2} + (mc

^{2})

^{2}2. Heating a body do increases its mass but not its kinetic energy.

3. Electromagnetic radiation going in a specific direction don't have mass; but if you consider a system of two beams of light going in two different directions, then this system do really have mass (it's weird, I know).

4. The total energy of a body *is not* proportional to its mass, in general; it is only if the body remains stationary. The formula E = mc

^{2} should be

canceled from the books (especially those for medium and high school students) and replaced with the one I've written up. Infact E = mc

^{2} is just the particular case of

E

^{2} = (cp)

^{2} + (mc

^{2})

^{2} when momentum p is = 0.