§1Mass, weight and density
Three quantities are constantly confused and must be kept apart: mass is the amount of matter (kilograms), weight is the force gravity exerts on it (newtons), and density is mass per unit volume (kg/m³).
The Measuring Units page makes the mass–weight distinction general; here it becomes practical. Density is the bridge from geometry to mass: multiply a part’s volume by its material’s density and you have its mass, then multiply by g = 9.806 65 m/s² for its weight. Every weight estimate on a drawing, every crane and lifting calculation, every shipping figure runs through density — so the density of the material you are working in is the single most-used number in this section.
Contents§2Density of the engineering metals
The common structural metals span a wide range — a factor of six from magnesium to lead — and that spread drives most material-selection decisions where weight matters.
| Material | Density | Note |
|---|---|---|
| Magnesium | 1740 | lightest structural metal |
| Aluminium | 2700 | ~⅓ the density of steel |
| Titanium | 4500 | light and strong |
| Cast iron | 7200 | |
| Steel | 7850 | the reference metal |
| Brass | 8500 | Cu + Zn |
| Copper | 8960 | |
| Lead | 11 340 | heaviest common metal |
| Water at 1000 kg/m³ is the natural yardstick: a material’s density divided by 1000 is its specific gravity, so steel’s specific gravity is 7.85 — it sinks nearly eight times as fast to the bottom as the same volume of water weighs. | ||
§3Weight from a drawing
The everyday calculation: turn a dimensioned part into a weight in two steps — volume from geometry, then mass and weight from density.
A steel bar Ø50 mm × 1000 mm long. Volume = (π/4) × 0.050² × 1.000 = 1.963 × 10⁻³ m³ (1963 cm³). Mass = 7850 × 1.963 × 10⁻³ = 15.41 kg. Weight = 15.41 × 9.806 65 = 151 N. Swap the material to aluminium (2700 kg/m³) and the same bar weighs only 5.3 kg — the density ratio, 2700/7850, carried straight through. This is why a weight estimate needs nothing more than the geometry and one density figure.
§4Specific heat
Specific heat is the energy needed to raise one kilogram of a material by one degree. It governs how much a furnace, a quench bath or a cutting zone heats up.
To raise 10 kg of steel (c ≈ 490 J/kg·K) by 200 °C takes Q = 10 × 490 × 200 = 980 kJ. The same mass of aluminium (c ≈ 900 J/kg·K) needs 10 × 900 × 200 = 1800 kJ — 1.84 times as much, because aluminium stores far more heat per kilogram. Water’s specific heat, 4186 J/kg·K, is higher still, which is exactly why water is the standard coolant and quenchant: it soaks up heat cheaply and in quantity.
§5Thermal expansion
Materials grow when heated. The coefficient of linear thermal expansion α gives the fractional length change per degree, and it matters wherever parts are fitted, heated or must hold a dimension.
Over one metre and a 100 °C rise, steel grows 11.7 × 1 × 100 = 1.17 mm; aluminium, at α ≈ 23 µm/m/°C, grows 2.30 mm — twice as much, the source of many bimetallic and clearance problems. The same coefficient underlies shrink and interference fits (heat the hub, drop it over the shaft, let it grip on cooling), the expansion gaps in long structures, and the 20 °C reference temperature at which the Dimensioning pages define every measurement. It is the same α used to compute thermal stress on the Strength of Materials page.
Contents§6Melting and the elements
A material’s melting point sets the ceiling on its service temperature and the floor for casting, welding and heat treatment.
| Metal | Melting point |
|---|---|
| Lead | 327 |
| Zinc | 420 |
| Magnesium | 650 |
| Aluminium | 660 |
| Copper | 1085 |
| Iron | 1538 |
| Titanium | 1668 |
| Tungsten | 3422 |
| Alloys melt over a range rather than at a single point, softening across a band between solidus and liquidus — which is why a plain metal has one figure here but a steel or a bronze has two. Tungsten’s extreme melting point is why it serves as lamp filament and electrode. | |
§7Quick reference
The working core of the page on one card rack.
Three quantities
mass (kg) · weight (N) · ρ (kg/m³)
W = m g
Weight
mass = ρ × volume
steel ρ = 7850
Heat
Q = m c ΔT
steel c ≈ 490 J/kg·K
Expansion
ΔL = α L ΔT
steel α ≈ 11.7 µm/m/°C
Yardstick
water ρ = 1000
SG = ρ/1000