Me

Elastic and plastic behaviour Cambridge International AS & A Level Physics revision

Not started

Learn it

In plain words

Stretch a rubber band a little and it springs back: that is elastic. Bend a paper clip too far and it stays bent: that is plastic. The work you do stretching something elastically is stored in it, ready to come back out, which is how a catapult works.

Three things to know

  1. Elastic deformation: the object returns to its original length when the load is removed. Plastic deformation: it is left permanently stretched.
  2. The elastic limit is the point beyond which the deformation becomes plastic.
  3. The area under a force–extension graph is the work done. Up to the limit of proportionality that work is stored as elastic potential energy (also called elastic strain energy): E = ½Fx = ½kx².

Worked example

A spring of spring constant 200 N/m is stretched by 0.050 m. Find the elastic potential energy stored in it.

  1. E = ½kx².
  2. = ½ × 200 × 0.050².
  3. = 0.25 J.

Tips and tricks

  • ½Fx only works for the straight-line part of the graph. Where the graph is curved, estimate the area by counting squares.
  • Double the extension and the stored energy goes up four times, since it depends on x².
5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Elastic and plastic behaviour: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

  1. What does the area under a force–extension graph represent?
    • the Young modulus
    • the stress
    • the work done (the answer)
    • the spring constant

    For the straight part it is ½Fx.

  2. A wire is stretched beyond its elastic limit, and then the load is taken off. What happens?
    • It stays longer than it was. (the answer)
    • It returns to its original length.
    • It always snaps.
    • It becomes shorter than it was.

    Beyond the elastic limit the deformation is plastic.

  3. A spring stores 0.50 J when stretched by 0.10 m. How much does it store when stretched by 0.20 m, within the limit of proportionality?
    • 0.25 J
    • 1.0 J
    • 2.0 J (the answer)
    • 4.0 J

    Energy depends on x², so twice the extension stores four times the energy.

  4. What is elastic deformation?
    • deformation that disappears when the load is removed (the answer)
    • deformation that is permanent
    • deformation that breaks the object
    • deformation caused only by compression

    The object goes back to its original size and shape.

  5. A 20 N force stretches a spring by 0.10 m, within the limit of proportionality. How much energy is stored?
    • 0.5 J
    • 1.0 J (the answer)
    • 2.0 J
    • 200 J

    ½ × 20 × 0.10.

Things you can type

Or go straight to

Or browse a shelf