Railway Science (Physics-Units and Measurements)

Relative density - The ratio of density of a substance to the density of the standard substance. Usually water at 4° C is used as a standard for a liquid or solid and air for a gas. It is a unitless quantity .Some other quantities which have No Units - deformity, Specific gravity.

Invented by William Gilbert. Periscope (Hippolyte Marie Davy) - A periscope works on the basis of laws of reflection of light. Endoscope (Philipp Bozzini) - A medical device used for examining the interior of a body cavity or organ. It works on the principle of Total Internal Reflection. Kaleidoscope (David Brewster) - Optical device consisting of mirrors that reflect images of bits of coloured glass in a symmetrical geometric design through a viewer.

Scientific Instruments: Hygrometer - Measure the amount of humidity in the atmosphere. Lactometer - Measure the purity of milk. Endoscope - Looks at the body's interior organs. Fathometer - Computes the ocean's depth. Galvanometer - Measures small- amplitude electric currents. Hydrophone - Gauges underwater sound.

A device used to detect and measure ionizing radiation (Alpha, Beta, and Gamma radiation). Spectrometer - Used to measure the variation of a physical characteristic over a given range.

Amount of radiation emitted by a surface in a given direction per unit solid angle per unit area of that surface normal to the mean direction in space. Quantity and Unit: Permeability - Henry per metre (H/m) or newton per ampere square (N/A² ), Electrical conductivity - Siemens per metre (S/m), Permittivity - Farad per metre (F/m).

The energy derived from electric potential energy or kinetic energy of the charged particles. SI unit of energy - Joule (J). Relationship between commercial and SI units of energy is : 1 kWh = 1kW × 1h = 1000W × 1h = 1000(J/s) × 3600 s = 3.6 × 10⁶ J . CGS unit of energy - erg. 1 erg = 10⁻⁷ J. Kilowatt - A unit to measure Power (1 KW = 1000 Watts).

The rate of − change in velocity is called acceleration. It is denoted by the symbol ‘a’. Its unit is metre/second² .
Acceleration = 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦/𝑡𝑖𝑚𝑒.  It is a vector quantity. The rate of change of acceleration is called ‘jerk’.

SI unit of other quantities: Area (m²), Density (kg m⁻³), Velocity (ms⁻¹), Force (N), Surface energy (joule/m²), Power of lens (Dioptre), surface tension (N-m⁻¹), Angular frequency (rad s⁻¹), Moment of inertia (kg-m²), specific gravity (no unit).

Total number of wave cycles completed in one second. Sound Waves are Longitudinal waves. The frequency determines the shrillness or pitch of a sound. Other Standard Units: metre per second - Speed. Newton - Force. Metre - Length.

Other units of length: One Millimeter = 1 × 10⁻³ meters. One Micrometer = 1 × 10⁻⁶ meters. One Nanometer = 1 × 10⁻⁹ meter. One Angstrom = 1 × 10⁻¹⁰  meters. One fermi = 10⁻¹⁵ meters.