A Level Physics Practice Exam

What is the limit of proportionality in a stress-strain graph?

1. The maximum stress that can be applied without breaking

2. The point beyond which the material behaves elastically

3. The point at which deformation becomes irreversible

4. The point at which the strain is zero

The correct answer is: The point beyond which the material behaves elastically

The limit of proportionality in a stress-strain graph corresponds to the point beyond which the material no longer obeys Hooke's Law, which states that stress is directly proportional to strain within the elastic limit of the material. Up to this point, the relationship between stress and strain is linear, and any deformation that occurs is reversible; when the load is removed, the material returns to its original shape. Beyond this limit, the proportional relationship breaks down, and the material starts to exhibit non-linear behavior, indicating that it is no longer behaving elastically. Therefore, identifying this point is crucial for understanding the elastic limit of a material and predicting how it will respond to loads applied to it.