Describe the difference between elastic and plastic deformation…

The idea

When a stretching force is removed, an object may return to its original shape or stay permanently changed. Elastic deformation occurs when the object returns to its original length and shape once the stretching force is removed; the material behaves elastically, as a rubber band or a spring does within its elastic limit. Plastic deformation occurs when the object is permanently distorted and does not return to its original shape after the force is removed; a metal paperclip bent too far or a piece of plastic stretched beyond its limit shows plastic behaviour. The key distinction is reversibility: elastic deformation is reversible, plastic deformation is not. A material may behave elastically for small forces and plastically once the force exceeds its elastic limit.

What earns the marks

• States that elastic deformation is reversible when the stretching force is removed.
• States that plastic deformation is permanent and the object does not return to its original shape.
• Contrasts the two by referring to whether the object regains its original length or shape.
• Applies the distinction to a named material or object such as a spring, rubber band or bent paperclip.

Common mistakes and how to put them right

• Confuses elastic with plastic and claims elastic deformation is permanent.

  Put it right by linking elastic to returning to the original shape.

• Describes plastic deformation as temporary.

  Put it right by stating that the distortion remains after the force is removed.

• Omits the role of removing the force.

  Put it right by explaining what happens once the stretching force is taken away.

Exam tips

• Use the words reversible and permanent to make the contrast explicit.
• Refer to what happens after the force is removed, since that is the defining test.
• Give a concrete example for each type to support your description.