Systems of Units:
- C.G.S (Centimetre Gram Second): It is also called the French System or Metric System, Introduced on 01 April 1956. In this system, length is measured in Centimetre, Mass in Grams and Time in Seconds.
- F.P.S (Foot Pound Second): It is the British system. In this system, length is measured in feet, Mass in pounds and Time in Seconds.
- M.K.S (Metre Kilogram Second): It is the American system. In this system, length is metres, Mass in kilograms and Time in Seconds.
- S.I (International System of Units): This system was adopted worldwide in 1960 as an SI system. India adopted the SI system of units on 01 April 1957. There are 07 fundamental units and 02 complementary units.
Fundamental Units:
- From 20 May 2019, the definitions of 04 fundamental units (kilogram, ampere, Kelvin and Mol) out of 07 fundamental units have been changed, now all 07 units have stable and constant values.
| Fundamental Physical Quantity | Units (SI) | Symbol | New Definition (20 May 2019) |
| Mass | kilogram | kg | Now kilogram will be defined by the Plank constant |
| Length | metre | m | – |
| Time | second | sec | – |
| Electric Current | Ampere | A | Now Ampere will be defined by the elementary electric charge (e) |
| Temperature | Kelvin | K | Now Kelvin will be defined by Boltzmann’s constant |
| Amount of substance | mol | mol | Now mol will be defined by Avagadro’s constant |
| Luminous intensity | Candela | cd | – |
Complementary or derived units:
| Fundamental Physical Quantity | Units (SI) | Symbol |
| Angular Displacement | Radian | rad |
| Solid Angle | Steradian | sr |
Important Physical Quantities and their Units:
| Physical Quantity | Unit |
| Charge | Coulamb (q) |
| Potential Difference | Volt (V) |
| Resistivity | Ohm-Meter |
| Resistance | Ohm (Ω) |
| Conductance | Mho=ohm-1 |
| Conductivity | siemens |
| Impedance | ohm |
| Capacitance | Farad |
| Inductance | Henry |
| Luminous Intensity | Candela |
| Luminous Flux | Lumen |
| Magnetic Flux | Waber |
| Magnetic Field Intensity | Tesla or Gause |
| Power of lens | Diopter=m-1 (m=focal length, always in meters) |
| Loudness of Sound | decibel (dB) |
| Radioactivity | Becquerel (Bq) |
| Nuclear radius | Fermi |
| perceived loudness of sound | Sone |
| Light | Photon |
Important dimensional formulae and units:
| Quantity | unit | dimensional formula |
| Force | newton | [MLT-2] |
| Friction | Newton | [MLT-2] |
| Tension | newton | [MLT-2] |
| thrust | newton | [MLT-2] |
| Torque | newton-meter | [ML2T-2] |
| Work | Joule or Newton-Meter | [ML2T-2] |
| Energy | Joule | [ML2T-2] |
| Surface Energy | Joule | [ML2T-2] |
| Heat | Joule | [ML2T-2] |
| Power | Watt or Joule/sec | [ML2T-3] |
| Velocity or Speed | M/sec | [M0LT-1] |
| Acceleration | m/sec2 | [M0 LT-2] |
| Linear Momentum | kg.m/sec | [MLT-1] |
| Angular Momentum | kg.m/sec2 | [MLT-2] |
| Angular Velocity | Rad./sec | [M0L0T-1] |
| Angular acceleration | rad/sec2 | [M0L0T-2] |
| Plank’s constant | joule.sec | [ML2T-1] |
| coefficient of viscosity | newton.sec/m2 | [ML0T-1] |
| Frequency | sec-1 | [M0L0T-1] |
| Gas constant | joule/mol.k | [ML2T-2K-1] |
| Latent Heat | joule/kg | [M0L2T-2] |
| Moment of inertia | kg.m2 | [ML2T0] |
| Pressure | Newton/m2 or Pascal or bar or atm |
[ML-1T-2] |
| universal gravitational constant | Nm2/kg2 | [M-1L3T-2] |
| Impulse | kg.m/sec or Ns | [MLT-1] |
| Modulus of Bulk/young/ of elasticity |
N/m2 or Pa | [ML-1T-2] |
| Stephen’s constant | J/K4.sec.m2 | [M1L0T-3K-4] |