Q.What are the advantages of S.I.?

Measurement is where physics touches the real world: every physical quantity is reported as a number × a unit, and the unit is the agreed-upon standard the number counts. Change the unit and the number changes with it — 2.5 m and 250 cm are the same length. Getting the units and their conversions honest is the foundation the rest of the paper stands on.

1 — Fundamental vs derived quantities. A handful of quantities are chosen as base (fundamental); every other quantity is derived by combining them. The SI system fixes seven base units: the metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance) and candela (luminous intensity). Speed, force, energy, charge and the rest are derived — e.g. the newton is kg·m·s⁻². A frequent trap is treating a derived quantity (like charge) as base, or forgetting that current, not charge, is the SI base electrical quantity.

2 — Systems of units. Before SI there were CGS (centimetre-gram-second), FPS (foot-pound-second) and MKS (metre-kilogram-second). SI is the modern, coherent and rationalised extension of MKS. Knowing which unit belongs where — dyne and erg are CGS, the poise measures viscosity in CGS — heads off mismatched-system errors.

3 — The 2019 SI redefinition. Since 2019 the base units are no longer tied to physical artefacts but to fixed exact values of seven constants: the caesium hyperfine frequency ΔνCs, the speed of light c, the Planck constant h (which now fixes the kilogram, retiring the platinum-iridium prototype), the elementary charge e (fixing the ampere), the Boltzmann constant k_B (the kelvin), the Avogadro number N_A (the mole) and the luminous efficacy K_cd (the candela).

4 — Prefixes, and practical / astronomical / atomic units. SI prefixes scale a unit by powers of ten — giga (10⁹), mega (10⁶), kilo (10³) … milli (10⁻³), micro (10⁻⁶), nano (10⁻⁹), pico (10⁻¹²). For area and volume the prefix power is squared or cubed (1 cm² = 10⁻⁴ m², 1 mm³ = 10⁻⁹ m³) — a classic slip. Convenient non-SI units survive for scale: the light year (9.46×10¹⁵ m), parsec (3.09×10¹⁶ m ≈ 3.26 ly) and astronomical unit (1.496×10¹¹ m) for astronomy; the ångström (10⁻¹⁰ m), fermi (10⁻¹⁵ m), atomic mass unit (1.66×10⁻²⁷ kg) and barn (10⁻²⁸ m²) for the atomic and nuclear world. …