1. The graphs below shows the behaviour of three springs after loads are attached. For each spring:
(a) Describe what material the spring is made from;
(b) Describe its elasticity;
(c) And whether or not it obeys Hooke’s law.
Force (N)
Extension (m)

2. 2.4 Young Modulus
Topic 2
Materials

3. Solids
When a spring is squashed and shortened, the force is described as a compressive force.
When a spring is stretched and made long, the force is described as a tensile force.
When a compressive force or tensile force is applied, the spring gets deformed. The ability to go back to its original shape depends on the elasticity of the spring.
If it goes back to its original position, then it obeys Hooke’s law.

4. Stress & Strain
All springs have a surface area. When a load is attached on a spring, it feels a pressure. This pressure is called stress.
σ =
Force
Area
Stress is measured in Pascal's.
When a load is attached on a spring, the length increases.
The extension of the spring can be compared to the original length using ratios. This is called strain.
ε =
Extension
Original Length
It has no units of measurements.

5. Young Modulus
The ratio of stress over strain gives us the Young modulus.
Young modulus tells us how stiff a spring is. It is measured in Pascal's.

6. Stress and Strain Graph
On a strain vs stress graph, young modulus can be found from the gradient.

7. Spring follows Hooke’s law; it has elastic behaviour.
Elastic limit is reached, it is permanently deformed.
Spring shows plastic behaviour; little force causes big extension.
Spring has reached its ultimate tensile stress (UTS).
Spring has reached its breaking point and snaps.