A light year is a unit of which quantity?

Units and Measurements involve quantifying physical quantities. The SI (Systeme International) is the internationally accepted system with 7 base units: metre (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, kelvin (K) for temperature, mole (mol) for amount of substance, and candela (cd) for luminous intensity. Units and Measurements is an important Physics topic for SSC CGL, Railway, and UPSC.

Important Physical Quantities and SI Units: Force - Newton (N), Pressure - Pascal (Pa), Energy/Work - Joule (J), Power - Watt (W), Electric charge - Coulomb (C), Electric potential - Volt (V), Resistance - Ohm, Capacitance - Farad (F), Frequency - Hertz (Hz), Temperature - Kelvin (K) or Celsius (C). These units are frequently asked in SSC CGL, Railway NTPC, and Physics GK sections.

Important Unit Conversions: Length: 1 km = 1000 m, 1 m = 100 cm, 1 inch = 2.54 cm, 1 foot = 12 inches. Mass: 1 kg = 1000 g, 1 quintal = 100 kg, 1 metric ton = 1000 kg. Area: 1 hectare = 10,000 sq m, 1 acre = 4047 sq m. Speed: 1 km/hr = 5/18 m/s. Pressure: 1 atm = 101325 Pa = 760 mmHg. Temperature: C = (F-32) x 5/9, K = C + 273.15.

Scalar quantities have only magnitude (no direction): mass, length, time, speed, temperature, energy, volume, density. Vector quantities have both magnitude and direction: displacement, velocity, acceleration, force, momentum, electric field. Speed (scalar) vs Velocity (vector): Speed is distance/time, Velocity is displacement/time. These distinctions are important for Physics questions in SSC CGL, NDA, and Railway exams.

Important Measuring Instruments: Thermometer (temperature), Barometer (atmospheric pressure), Hygrometer (humidity), Lactometer (purity of milk), Seismograph (earthquakes), Speedometer (speed of vehicle), Altimeter (altitude), Ammeter (electric current), Voltmeter (voltage), Galvanometer (small current), Manometer (gas pressure), Anemometer (wind speed), Hydrometer (specific gravity of liquids).