Strength vs. Stiffness: The Core Distinction
Strength is a material's ability to withstand a load without breaking or permanently deforming. It measures the maximum stress a material can endure. In contrast, stiffness is a material's resistance to elastic deformation—how much it bends or stretches under a load. A stiff material changes shape very little when a force is applied.
Section 2: Measuring the Properties
Strength is typically measured in units of pressure, such as Pascals (Pa) or pounds per square inch (psi), and is determined by finding the ultimate tensile strength or yield strength of a material. Stiffness is quantified by the Young's modulus (or modulus of elasticity), which describes the relationship between stress (force per unit area) and strain (proportional deformation). A higher Young's modulus means a stiffer material.
Section 3: A Practical Example
Consider a glass rod and a rubber band. The glass rod is very stiff; it will not stretch easily. However, it is not very strong compared to metals; a moderate force will cause it to snap (brittle failure). The rubber band, on the other hand, is not stiff at all; it stretches easily under a small force. However, it has considerable strength, as it can be stretched a great deal before it breaks.
Section 4: Why This Distinction Matters
Understanding the difference between strength and stiffness is crucial in engineering and design. For a bridge beam, you need a material that is both strong (to support heavy loads without failing) and stiff (to prevent excessive sagging). For a car's suspension spring, you need a material with high strength but specific, lower stiffness to absorb bumps effectively.