1 speed of light
i dont know the exact way but it was some sort of mirrors reflecting the light back anf forth and by increasing the speed of turning it on and off till the light does not hit the last mirror and measuring the distance it took to get to the last mirror and multiply it by the speed the light was turned on and off and that would be a primitive way of measuring it.

2 once we split the atom we realised that energy and mass were interchangeable
a kilogramm of hydrogen is also a heap of pure energy. (the hydrogen bomb )
so once you get to the speed of light and the speed of the the electrons in the piece of mass they gain weight/energy and it will be a sort of kinetic energy store

as for the rest of the questions im a bit out of my depth here

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