Edexcel IGCSE Physics · Spec 1.22, 1.24
Deformation of Springs, Wires and Rubber
Covers investigate three deformable materials, Identify the limit of proportionality and Define elastic behaviour.
Physics revision video
Deformation of Springs, Wires and Rubber
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Explained
Stretching three materials, and where the line stops being straight
Hang a mass on a spring and it stretches. Add more and it stretches further. The investigation is to find out exactly how the extension depends on the force, for three different materials.
The method
Clamp the spring, wire or rubber band vertically and measure its original length with no load.
Add a known mass, wait for the oscillations to stop, and measure the new length. Extension is the new length minus the original length, so it is a change rather than a measurement in itself. Repeat in equal steps, recording the force and the extension each time.
Waiting for the oscillations to stop matters, because a moving pointer cannot be read accurately. Reading at eye level avoids parallax error, and taking a reading as you unload as well as as you load lets you check whether the material returned to its original length.
The three materials
A helical spring gives a straight line through the origin over a good range. Extension is directly proportional to force, which is Hooke's law.
A metal wire also gives a straight line, but it stretches much less for the same force, so the line is far steeper, and it reaches its limit sooner.
A rubber band gives a curve rather than a straight line, and does not obey Hooke's law at any point. It also follows a different path when unloading than when loading.
The limit of proportionality
This is the point beyond which the graph is no longer straight, so extension is no longer proportional to force and Hooke's law no longer applies.
It is not the same as the elastic limit, though they are close together. Elastic behaviour means the material returns to its original length when the force is removed. Beyond the elastic limit it stays permanently deformed, and a spring stretched that far never goes back.
So a material can be stretched beyond the limit of proportionality and still be elastic. The graph has curved, but the spring will still return.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics multiple choice question asks which energy store fills as a spring is stretched, and the mark scheme explains each wrong option individually: there is no transfer from a chemical store, the gravitational store decreases rather than increases, and the spring is stationary in both cases so no kinetic store is involved.
Those three explanations are a useful checklist. Stretching fills the elastic store; the mass falling empties the gravitational store; and nothing moving means no kinetic store.
On the longer question that follows, about a mass bouncing on a spring, the mark scheme wants the transfers traced in order and reversed on the way back, and credits the idea that the amplitude of the oscillations decreases over time. It allows slowing down to mean that the maximum speed decreases, and accepts a reference to the total energy of the system reducing.
Energy has not been destroyed there. It has been transferred to the thermal store of the surroundings through air resistance and internal friction in the spring, which is the same dissipation that appears everywhere in this topic.
Reading the graph
Force is usually plotted on the vertical axis and extension on the horizontal. The straight section shows proportionality, and the point where it starts to bend is the limit of proportionality.
Be careful with the gradient. With force on the vertical axis, a steeper line means a stiffer material, because more force is needed for the same extension. If the axes are the other way round, the reverse is true, so check which quantity is where before saying anything about steepness.
Spec 1.22, 1.24
What you need to know
- Investigate three deformable materials
- Identify the limit of proportionality
- Define elastic behaviour
Active recall
Quick check
Answer each question before opening the answer.
Which three materials should be compared?
A helical spring, metal wire and rubber band
Where is the limit of proportionality?
Where the force-extension graph stops being straight
What makes behaviour elastic?
The material recovers its original shape and length when the force is removed
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