1. To understand deformation of materials

2. Discuss the physics of bungee jumps
What factors do the organisers of a bungee jump need to take into account to ensure the safety of the individual?
Discuss the physics of bungee jumps

3. Elastic and plastic deformation
Solids are made of huge numbers of tightly packed atoms, so when a solid is compressed (squeezed) or subject to tension (stretched) the spacing of the atoms is altered.
The forces between the atoms are small, but because there are so many of them, a large force is necessary to gain an appreciable change in the size or shape of the solid.
Usually when the force is removed, the solid returns to its original shape.

4. Key definitions
Elasticity is the property of a body to resume its original shape or size once the deforming force or stress has been removed
Deformation is the change in shape or size of an object. If the material returns to its original shape then the deformation is elastic, if not then the deformation is plastic

5. Examples of elastic materials
These materials all change shape when a reasonable force is applied, but will return to their original shape and size when the force is removed

6. Plastic behaviour
Plastic behaviour can be thought of as the opposite of elastic behaviour
Once the deforming force is removed, the material remains stretched. This is permanent.
Plastic deformation occurs when the material has passed its elastic limit – the force beyond which the atoms can no longer return to their original arrangement

7. Describing elastic and plastic behaviour
Different materials respond differently to tensile forces.
The loading and unloading curves may not be the same

8. Describing elastic and plastic behaviour for a metal wire
The wire obeys Hooke’s Law up to the Elastic limit (A). The unloading graph will be identical for forces less than the elastic limit.
Beyond the elastic limit the unloading graph is parallel to the loading graph.
The wire is permanently extended after the force is removed, it has suffered plastic deformation.
F [N]
x [m]
Elastic Limit
Loading
Unloading
Permanent extension e } A

9. Rubber
Thermal energy is released when the material is loaded then unloaded, represented by the area inside the hysteresis loop.
Rubber bands do not obey Hooke’s law. The band will return to its original length after the force is removed
The loading and unloading graphs are curved and are different
The ‘loop’ formed by the loading and unloading curves is called a hysteresis loop.
More work is done when stretching a rubber band than during the unloading.

10. Polythene A polythene strip does not obey Hooke’s Law. Force / N
The loading curve is similar to a rubber band but:
Thin strips of polythene are very easy to stretch and suffer plastic deformation under relatively little force
Force / N
Extension / m

11. What have you done so far to ensure:
You have understood incorrect answers on homeworks or in class?
Your notes are ordered logically?
You are learning definitions?
You are reducing ‘silly’ mistakes?