why the simple mass kilogram is kept in 0 celcious?

Answer 1

The main reason is that objects expand when their temperatures increase. Air makes all objects feel slightly lighter due to buoyancy and this buoyancy depends upon an object's volume. Therefore a slightly warmed object will appear to weigh less due to buoyancy because it is slightly larger.

It's very a small difference, but when you're dealing with the level of accuracy they're trying to attain, every little bit counts, and you want to be as consistent as possible.

Another benefit would be that the rate of oxidation would be reduced. But since the standard kg is stored in triple-sealed jars and made of very oxidation resistant materials (a platinum-iridium alloy), I wouldn't expect this to matter much.

Interestingly, this alloy was chosen also because of its high density, and therefore small size, again to minimize the buoyancy of air.

By the way, the effects of relativity would not be measurable.

Answer 2

Both Saleh_Sep's and Thomas G's answers are perfectly correct, though quite different. I think they're both right. The idea is to rigorously reduce any variations in the standard Mass's environment.

I once used an ultra-precision balance kept inside a sealed chamber and maintained at a constant "atmospheric" pressure to minimize variable buoyancy effects. Temperature was also constant, though not zero degrees.

Kinetic energy literally is the same as mass. That's why light can be bent by gravity....predicted by Einstein and now called a gravitational lens by astronomers.

Answer 3

according to famous formula (E=mc^2) if we increase temperature of a mass we have increase of energy :
dQ=dE
according to first formula
dE=c^2(dm)
in classic physics dm=0 but in modern physics it exists. so if you increase temperature of a mass it will find little extra mass.
of course it is so little.