Elastic and plastic behaviour Edexcel International A Level Physics revision
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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
- Elastic deformation: the object returns to its original length when the load is removed. Plastic deformation: it is left permanently stretched.
- The elastic limit is the point beyond which the deformation becomes plastic.
- 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.
- E = ½kx².
- = ½ × 200 × 0.050².
- = 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.
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.
What does the area under a force–extension graph represent?
For the straight part it is ½Fx.
A wire is stretched beyond its elastic limit, and then the load is taken off. What happens?
Beyond the elastic limit the deformation is plastic.
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?
Energy depends on x², so twice the extension stores four times the energy.
What is elastic deformation?
The object goes back to its original size and shape.
A 20 N force stretches a spring by 0.10 m, within the limit of proportionality. How much energy is stored?
½ × 20 × 0.10.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 7 marks. Write your answers on paper, then check them.
Elastic and plastic behaviour
Edexcel International A Level Physics WPH · 7 marks · papermunch.org
Name ______________________________ Date ______________
A 12 N force stretches a spring by 0.060 m, within the limit of proportionality. Find the energy stored.[2]
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0.36 J. ½ × 12 × 0.060.
State the difference between elastic and plastic deformation.[2]
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After elastic deformation the object returns to its original shape when the load is removed. After plastic deformation it stays permanently deformed.
A catapult stores 2.0 J of elastic potential energy and fires a stone of mass 0.010 kg. Assuming all the energy is transferred to the stone, find its speed.[3]
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20 m/s. v = √(2 × 2.0 ÷ 0.010).
Answers: Elastic and plastic behaviour
- 1. 0.36 J. ½ × 12 × 0.060.
- 2. After elastic deformation the object returns to its original shape when the load is removed. After plastic deformation it stays permanently deformed.
- 3. 20 m/s. v = √(2 × 2.0 ÷ 0.010).