What is elastic potential energy?
Energy stored when a solid is stretched or compressed.
What happens if a solid is strained elastically or plastically?
Elastically: Energy can be fully recovered.
• Plastically: Some energy is lost, not recoverable.
What does the area under a force–extension graph show?
Work done
What happens to a metal wire when it is stretched ?
It obeys Hooke’s law; extension is proportional to force, and it returns to its original length when unloaded.
What is the elastic limit of a metal wire?
No longer obeys Hooke’s law and returns to its original length.
What happens to a metal wire beyond its elastic limit?
It undergoes plastic deformation: it is permanently extended after the force is removed.
How does a rubber band behave under tension?
It does not obey Hooke’s law, and the extension is not proportional to force.
What happens to a rubber band when the force is removed?
It returns to its original length, but loading and unloading graphs are curved and different.
What is a hysteresis loop?
The loop formed by the difference between loading and unloading curves for materials like rubber.
What does the area inside a hysteresis loop represent?
Thermal energy released during loading and unloading.
Why does polythene strip not obey hookes law?
Thin strips of polythene easy to stretch
Suffer plastic deformation
under relatively small force
What is the yield point?
large increase in the extension of a material
stress is increased beyond the material’s elastic limit
Front:
What is the Ultimate Tensile Strength/fracture point?
Back:
• Maximum stress a material can withstand.
• Higher UTS = stronger material.
• After this point → material starts to weaken.
What happens is stress on the brittle material is increased?
Stretches slightly, further increase of stress = fracture
Large stress on a brittle material
Material will shatter
What is a ductile material?
• Stretches a lot before breaking.
• Behaves elastically up to the elastic limit.
• Beyond elastic limit → plastic deformation (permanent change).
• Can be shaped by stretching, rolling, hammering (e.g., copper, gold).
How does a ductile material behave under stress?
• Elastic region: returns to original shape.
• Plastic region: permanent deformation.
• Eventually → breaks.
What affects the behaviour of polymers?
Back:
• Temperature
• Molecular structure
(These determine how stretchy/tough/brittle they are.)
How does polythene behave under stress?
• Very stretchy and easy to deform.
• Quickly undergoes plastic deformation.
• Stretches a lot before becoming stiff.
• Snaps similar to a ductile material.
Front:
How does Perspex behave?
Back:
• Hard, tough plastic.
• Stretches elastically then snaps suddenly → brittle behaviour.
• When warmed → not brittle, can be moulded into
shapes.
• Used for contact lenses, dentures, artificial eyes, etc.
Ductile material
Can stretch and it stays stretched
Large plastic region
Malleable
Squash material and it stays squashed
Brittle
No plastic region just breaks
Glass, crisp
