Youngus

Meet Youngus, a steel rod who barely stretches under a heavy load.

Pull on a solid rod and it stretches a little. Stress σ is the force per unit area of the cross-section, and strain ε is the stretch divided by the original length. For small stretches the two are proportional, and their ratio is Young's modulus, E = σ/ε.

Steel has a Young's modulus of about 200 GPa. Hang a 10,000 N load from a steel rod 1 m long with a cross-section of 1 cm². The stress is 10,000 / 0.0001 = 10⁸ Pa, so the strain is 10⁸ / (2 × 10¹¹) = 5 × 10⁻⁴, a stretch of just 0.5 mm.

A rubber band is thousands of times less stiff, which is why it stretches so easily under a small pull.

Engineers use Young's modulus to choose materials for bridges, buildings and aircraft, making sure each part bends and stretches only as much as it safely should.