Density — mass per unit volume — is one of the most important material properties. It affects whether things float, how heavy a part will be, and how much material you need. Engineers select materials partly based on density-to-strength ratios.
Common Material Densities
Metals: Aluminum 2,700 kg/m³, Steel 7,850 kg/m³, Copper 8,960 kg/m³, Titanium 4,500 kg/m³, Gold 19,300 kg/m³, Platinum 21,450 kg/m³.
Plastics: Polyethylene (PE) 930–970 kg/m³, PVC 1,300–1,450 kg/m³, Nylon 1,130–1,150 kg/m³, PTFE (Teflon) 2,150 kg/m³.
Woods: Balsa 100–200 kg/m³, Pine 350–500 kg/m³, Oak 600–900 kg/m³, Ebony 1,100–1,300 kg/m³.
Liquids: Gasoline 730 kg/m³, Water 1,000 kg/m³, Seawater 1,025 kg/m³, Mercury 13,546 kg/m³.
Density in Design Decisions
Aerospace: titanium (4,500 kg/m³) preferred over steel (7,850 kg/m³) for a 43% weight saving at similar strength. Automotive: aluminum engine blocks save 40% weight vs cast iron.
Marine: materials must be less dense than seawater (1,025 kg/m³) to float, or the hull design must provide enough buoyancy. Wood floats (most types), metals sink — unless shaped to displace enough water.
Specific Gravity
Specific gravity = material density ÷ water density. It's dimensionless. SG of 2.7 means the material is 2.7× denser than water. Aluminum has SG ≈ 2.7, steel ≈ 7.85, gold ≈ 19.3.
Specific gravity is used in brewing (sugar content), gemology (identifying stones), and geology (mineral identification). A gold prospector uses specific gravity to distinguish real gold (SG 19.3) from pyrite "fool's gold" (SG 5.0).
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