Mechanical Properties of Solids
Class 11 · Physics
4 ideas in Class 11 Physics, explained in plain words, with games and quizzes to practise.
Stress and Strain
Part of: Deformation and Elasticity
A deforming push wakes an equal restoring push inside the body. Restoring push per unit area is stress: pulling gives tensile, squeezing gives compressive, sliding gives shearing, and all-round squeezing gives hydraulic. Each has a matching strain, which is a pure ratio with no units.
Why it matters. Every structure lives or fails by these quantities. Naming the right pair is the first step of any strength calculation.
Formulas
- Stress = force / area
- Strain = change / original size
Key words
- Stress
- : Restoring push per unit area, measured in pascal.
- Strain
- : Deformation as a pure ratio with no units.
- Tensile stress
- : Pulling stress that lengthens a body.
- Compressive stress
- : Squeezing stress that shortens a body.
- Shearing stress
- : Sliding stress that tilts a body.
- Hydraulic stress
- : All-round squeezing that shrinks volume only.
Try it in two minutes
- Stretch a rubber band and name the stress.
- Squash a sponge and name the change.
- Slide a book's top over its base and name the tilt.
Try at home. Sort home deformations into pull, push, slide and squeeze.
Hooke's Law and the Stress-Strain Curve
Part of: Deformation and Elasticity
Small loads stretch in proportion: double the stress, double the strain. The graph climbs straight, then bends at the yield point where full recovery ends. Further loading brings permanent set, an ultimate peak, then fracture. Close peak and break means brittle; a long gap means ductile.
Why it matters. Designers must stay inside the straight part. Reading the curve separates safe loads from ruins.
Formulas
- Hooke law: stress is proportional to strain (small loads only).
- Order on the curve: proportional, yield, plastic, peak, fracture.
Key words
- Hooke's law
- : Small stress grows in proportion to strain.
- Elastic limit
- : The largest load still allowing full recovery.
- Yield point
- : The point where full recovery ends.
- Plastic deformation
- : Leftover shape kept after unloading past yield.
- Brittle
- : Breaking near peak strength with little stretch.
- Ductile
- : Stretching far beyond peak before breaking.
- Elastomer
- : A material stretching hugely yet returning, like rubber.
Try it in two minutes
- Load a spring lightly and watch it return fully.
- Overload a paperclip and note the leftover bend.
- Compare a snapping biscuit with a stretching toffee.
Try at home. Test which home threads snap cleanly and which stretch first.
Elastic Moduli and Poisson's Ratio
Part of: Moduli and Real Structures
Each stress-strain pair owns a modulus: Young's for stretch, shear for slide, bulk for squeeze. Compressibility is the bulk modulus upside down. Pulling also thins the sides, and Poisson's ratio links that thinning to the stretch.
Why it matters. Tables of moduli choose ropes, slabs and hulls. One ratio predicts the sideways price of every stretch.
Formulas
- Y = tensile stress / tensile strain
- Shear modulus = shearing stress / shearing strain
- Bulk modulus = pressure / (volume change / volume)
- Poisson ratio = lateral strain / length strain
Key words
- Young's modulus
- : Stretch stiffness, tensile stress over strain.
- Shear modulus
- : Slide stiffness, shearing stress over strain.
- Bulk modulus
- : Squeeze stiffness, pressure over volume fraction lost.
- Compressibility
- : Volume fraction lost per unit pressure, the bulk modulus upside down.
- Poisson's ratio
- : Side thinning over length stretch, with no units.
Try it in two minutes
- Stretch a band and watch it narrow.
- Rank steel, glass and rubber by stiffness feel.
- Press a sealed syringe and feel bulk resist.
Try at home. Compare stretch of threads of different materials under one weight.
Stretching, Squeezing and Building
Part of: Moduli and Real Structures
Wire stretch follows force times length over area times Young's modulus, and the work sits stored as half stress times strain times volume. Ocean pressure squeezes water measurably; rope size follows load over yield strength with a safety margin; even mountains are capped by rock strength.
Why it matters. These formulas size every lift, dive and skyline. Small numbers here prevent large disasters.
Formulas
- Stretch = F L / (A Y)
- Stored energy = (1/2) x stress x strain x volume
- Needed area = load / yield strength
Key words
- Elongation
- : The stretch of a wire under load.
- Elastic potential energy
- : Work stored in a stretched body.
- Yield strength
- : The largest stress a rope or beam can carry and still recover.
- Safety factor
- : The extra margin engineers add beyond the minimum size.
Try it in two minutes
- Hang weights on a thread and plot stretch.
- Compute a rope size for a known load.
- Estimate ocean squeeze from depth and modulus.
Try at home. Hang a bag on a rubber band and predict its stretch.
What your child will practise
- Elastic Moduli and Poisson's Ratio: Candy Quick Fire (Quick Fire)
- Hooke's Law and the Stress-Strain Curve: Desert Puzzle (Clue Puzzle)
- Hooke's Law and the Stress-Strain Curve: Desert Quick Fire (Quick Fire)
- Stress and Strain: Space Quick Fire (Quick Fire)
- Stretching, Squeezing and Building: Ocean Puzzle (Clue Puzzle)
- Quiz: Elastic Moduli and Poisson's Ratio (8 questions)
- Quiz: Hooke's Law and the Stress-Strain Curve (8 questions)
- Quiz: Stress and Strain (8 questions)
- Quiz: Stretching, Squeezing and Building (8 questions)