1. Hookes Law The following topics will be discussed in this presentation: 1. Hookes law 2. Elastic behaviour of materials by stretching a spring and producing a graph of force versus extension 3. Plastic behaviour of materials

2. Hookes Law a b c d e f Hookes law: Extension produced in a material is proportional to the tensile force, within the elastic limit. In this test a spring is extended by an applied force. The extension produced is recorded and plotted against force. At the start of the test the extension is zero as no force is applied to the spring. Extension Force

3. Hookes Law 1 a b c d e f a 2 b The spring is extended by an applied force. Force 1 produces an extension which is plotted on Force versus Extension graph – i.e. point a. Extension Force Extension Force b a As the applied force is increased to Force 2, the extension increases too. This is shown by point b on the graph.

4. Hookes Law 3 a b c d e f a 4 d As the force is increased, the extension of the spring increases to mark c. Extension Force Extension Force b a b c c d On further increasing the force, the spring extends to mark d. The graph is a straight line showing elastic behaviour of spring.

5. Hookes Law a b c d e f a 4 e Extension Force Extension Force b a b c c d d If the force is removed at any point up to the elastic limit, the spring reverts back to its original shape and size 5 e f A further increase in force will cause a larger extension of the spring and its extension is no longer proportional to the force, hence the shape of the graph changes.

6. Hookes Law e Extension Force b a c d 5 e f If the force is removed now the spring does not revert back to its original shape and size. This change is called the permanent set. Permanent set