Science.
Glossary

The internal force per unit area within a material resisting an external load. Formula: σ = F/A, where σ is stress (in Pascals), F is the applied force (in Newtons), and A is the cross-sectional area (in m²). Types include tensile stress (pulling apart), compressive stress (pushing together), and shear stress (sliding forces).

The deformation of a material under stress, expressed as a ratio: ε = ΔL/L₀, where ΔL is the change in length and L₀ is the original length. Strain is dimensionless. Elastic strain is recoverable when the load is removed; plastic strain is permanent.

A measure of a material's stiffness — how much it resists deformation under tensile or compressive stress. E = Stress/Strain. Expressed in Pascals (Pa) or GPa. Steel: ~200 GPa. Aluminium: ~69 GPa. Rubber: ~0.01–0.1 GPa. A higher Young's modulus means a stiffer material that deforms less under the same load.

The stress level at which a material transitions from elastic (recoverable) to plastic (permanent) deformation. Below the yield strength, a material returns to its original shape when the load is removed. Exceeding the yield strength permanently deforms the component. Structural designs target stresses well below yield strength using safety factors.

The maximum stress a material can withstand before fracturing. UTS > Yield Strength. The difference between yield strength and UTS represents the material's ability to deform plastically before breaking — a desirable property in structural materials that gives visible warning before catastrophic failure.

The ratio of a structure's failure load to the maximum expected working load: SF = Failure Load ÷ Design Load. A safety factor of 3 means the structure must carry three times the design load before failing. Higher safety factors are used when consequences of failure are severe, material properties are uncertain, or loading conditions are variable.

Stress acting parallel to a surface or cross-section, as opposed to normal stress which acts perpendicular. Shear stress causes one layer of material to slide relative to adjacent layers. Common in beams, bolts, welds, and shafts transmitting torque. Symbol: τ (tau), formula: τ = F/A where F is the shear force and A is the shear area.

Mass per unit volume of a material. Formula: ρ = m/V. SI unit: kg/m³. Common equivalents: 1 g/cm³ = 1,000 kg/m³ = 1 kg/L. Water at 4°C: 1,000 kg/m³. Steel: ~7,850 kg/m³. Aluminium: ~2,700 kg/m³. Lead: ~11,340 kg/m³.

The ratio of a material's density to the density of water at 4°C (1,000 kg/m³). A dimensionless number — specific gravity of 7.85 means the material is 7.85 times denser than water. Materials with specific gravity below 1 float in water; above 1 they sink. Identical numerical value to density in g/cm³.

The fractional change in length per degree of temperature change. Unit: 1/°C (or 1/K). Common values: Aluminium 23.1×10⁻⁶/°C, Carbon steel 11.7×10⁻⁶/°C, Stainless steel 16.0×10⁻⁶/°C, Invar 1.2×10⁻⁶/°C, Glass 8.5×10⁻⁶/°C. A higher α means more expansion per degree of temperature change.

The increase in length of an object due to temperature increase. Formula: ΔL = α × L₀ × ΔT, where α is the coefficient, L₀ is original length, and ΔT is the temperature change. A 1-metre steel rod (α = 11.7×10⁻⁶) heated by 100°C expands by 1.17 mm. Relevant for precision manufacturing, structural engineering, and calibration.

A fluid's resistance to flow or deformation under shear stress. Dynamic viscosity (μ) is measured in Pascal-seconds (Pa·s) or centipoise (cP). Kinematic viscosity (ν) divides dynamic viscosity by density (m²/s or centistokes cSt). Higher viscosity = thicker, slower-flowing fluid. Used in lubricant specification (ISO VG grades, SAE grades for engine oils).

The fundamental relationship between voltage, current, and resistance in an electrical circuit: V = I × R. Voltage (V) in Volts, Current (I) in Amperes, Resistance (R) in Ohms (Ω). Rearranges to I = V/R and R = V/I. Applies to resistors and most conductors at constant temperature.

The electric potential difference between two points, driving current through a circuit. Measured in Volts (V). Analogous to water pressure in a pipe — the higher the voltage, the greater the force pushing electrons through the conductor. Named after Alessandro Volta.

The flow rate of electric charge through a conductor, measured in Amperes (A). One Ampere equals one Coulomb of charge passing a point per second. Direct current (DC) flows in one direction; alternating current (AC) reverses direction periodically. Named after André-Marie Ampère.

A material's opposition to the flow of electric current, measured in Ohms (Ω). Resistance converts electrical energy into heat. Resistors in series: R_total = R1 + R2 + R3. Resistors in parallel: 1/R_total = 1/R1 + 1/R2 + 1/R3. Resistance increases with temperature in most conductors.

The rate of electrical energy transfer, measured in Watts (W). P = V × I = I² × R = V²/R. A 100W light bulb on a 120V circuit draws approximately 0.83A. Power dissipated in a resistor becomes heat — a resistor rated at 0.25W running at 0.25W will have a significantly shortened service life.

The internationally standardised system of measurement. Base units: metre (length), kilogram (mass), second (time), Ampere (current), Kelvin (temperature), mole (amount), candela (luminosity). All other units derive from combinations of these base units. The standard for scientific and engineering work globally.

The meaningful digits in a measurement, indicating its precision. A measurement of 3.14 has three significant figures; 3.140 has four. When calculating, the result cannot have more significant figures than the least precise input. Engineering calculations typically work to 3–4 significant figures unless precision-critical.

A measure of how spread out values in a data set are from the mean. A low standard deviation means values cluster tightly around the mean; a high standard deviation means they are spread widely. Denoted σ for a population or s for a sample. For a normal distribution, approximately 68% of values fall within ±1 standard deviation of the mean.

The sum of all values divided by the number of values. The most common measure of central tendency. Sensitive to outliers — a single extreme value can significantly shift the mean. For skewed data sets, the median may be more representative.

The permissible range of variation in a manufactured dimension. A shaft specified as 25.000 ± 0.025 mm has a tolerance of 0.050 mm (total range). Tighter tolerances increase manufacturing cost and difficulty. Engineering drawings specify tolerances for every critical dimension.