General understanding: 1. What is the density of a material?

### General understanding: 1. **Density** is the ratio of mass to volume ($ρ = m/V$). 2. The units are $kg/m^3$. Low density is needed in aircraft, spacecraft, and lightweight equipment. 3. Water: $\approx 1000 kg/m^3$, Aluminium: $\approx 2700 kg/m^3$, Steel: $\approx 7850 kg/m^3$. 4. Stiffness measures resistance to elastic deformation. Important when precise dimensions must be maintained under load (e.g., bridge beams, machine frames). 5. **Young's modulus** is the ratio of stress to strain (describes stiffness). 6. **Strength** is the ability of a material to withstand applied loads without failing (breaking or permanent deformation). 7. **Yield strength** is the stress at which a material begins to deform plastically. Fracture strength is higher because the material undergoes strain hardening before it finally breaks. 8. **Ductility** is the ability to deform under tensile stress (e.g., copper, aluminium, gold). Brittle materials break with little deformation (e.g., glass, cast iron). 9. **Toughness** is the ability to absorb energy and deform plastically before fracturing. 10. Steel properties: a) Springs: High yield strength and high elastic limit (resilience). b) Car body parts: Formability (ductility) and impact resistance. c) Bolts and nuts: High tensile strength and hardness. d) Cutting tools: High hardness and wear resistance. 11. Aluminium is used in aerospace, transport, and packaging due to its low weight-to-strength ratio. ### Exercise 3.3. Find the words: 1. Density (quantity of mass per unit volume) 2. Kilogram per cubic meter 3. Measure of stiffness (resistance to deformation) 4. Young modulus (ratio of applied force per unit area to partial elastic deformation) 5. Rigid construction 6. Compressive strength 7. Ductility (ability of material to deform without breaking) 8. Toughness (to absorb energy through deformation) 9. Inversely proportional to the square of the defect size 10. Creep (gradual change of shape) 11. Elevated temperatures 12. High tensile stresses ### Exercise 3.4. Translate into English: 1. Density is measured in kilograms per cubic meter. 2. Most materials have a higher density than water and sink in water. 3. Density of materials is very important, especially in aviation. 4. Young's modulus is the ratio of applied force to the elastic deformation of a given material. 5. The stiffer the metal, the less it deforms under load. 6. When a metal is stretched, it first yields, that is, it deforms plastically. 7. Lead, copper, aluminium, and gold are the most malleable metals. 8. Creep resistance is a very important property of materials used in aircraft engines.