Question

how are fundamental quantities different from derived quantities?

Answer 1

Fundamental quantities can only be measured -- we have no way of deducing them from other measurements and theory. For example, the mass and charge on the proton and electron can only be measured. There is no way to calculate them from first principles. However, once we know them, we can calculate the energy of an electron when it is orbiting a proton (in an H atom). So that's not a fundamental quantity, it's something we can derive.

Answer 2

fundamental units are chosen as principle in a primary standard, the others are derived in a theory as secondary standards. There can be different systems for example: cgs (centimeter, gram, second) or mks (meter, kilogram, second). Or for example the SI (systeme internationale) which has the second and the speed of light as definitions. the second is the time required for 9192631770 periods of the microwave that stimulates an electron in a cesium atom in an atomic clock, and the speed of light is absolute because we are all confident of Einsteins specific theory of relativity STOR:299792458 m/s and we measured it. (How did he, with the moon reflector placed by the lunanauts? Can anyone answer me that?)

Answer 3

I believe the earliest speed of light estimates were based on the retardation of the eclipses of Jupiter's moons as Jupiter moved away from the Earth. Later measurements used mirrors on hills and such.

Answer 4

thanx carl , yeah i knew that. Great men these astronomers, star gazers.