Question

What does Hooke’s Law essentially define?
(A) Stress
(B) Strain
(C) Yield Point
(D) Elastic Point

Answer 1

Hint : Elastic behaviour in a body does not allow permanent deformation, and the body retains to its original shape and size. Yielding, also called plastic behaviour, occurs beyond the elastic limit.

Formula Used: We will be using the following formula in the solution,
 $\Rightarrow F\; = -k \cdot x $ where $ F $ is the force, $ x $ is the length extended and $ k $ is the spring constant.

Complete step by step answer
Sir Robert Hooke published the Hooke’s Law, which states that, within the elastic limit of a material, the strain of the material is proportional to the stress applied on it.
Elastic point is the maximum stress that can be applied on a body before the onset of permanent deformation. It marks the maximum limit of stress applied on a body beyond which it shows plastic behaviour.
Stress applied results in deformation of a body. On removal of stresses less than the elastic limit, the material resumes its original size and shape. Once the stress applied exceeds the elastic limit, the body flows. This is also called yielding of a body, and such behaviour is called plastic behaviour. For such materials the elastic limit marks the end of elastic behaviour and the beginning of plastic behaviour.
Yield point marks the minimum amount of stress in a solid that is required to produce permanent deformation. The yield point therefore, marks the end of elastic behaviour and the beginning of plastic behaviour. Once stresses that are less than the yield point are removed, the material returns to its original shape, that is, it shows elastic behaviour.
Elastic point is the maximum stress that can be applied on a body before the onset of permanent deformation. It marks the maximum limit of stress applied on a body beyond which it shows plastic behaviour.
Stress applied results in deformation of a body. On removal of stresses less than the elastic limit, the material resumes its original size and shape. Once the stress applied exceeds the elastic limit, the body flows. This is also called yielding of a body, and such behaviour is called plastic behaviour. For such materials the elastic limit marks the end of elastic behaviour and the beginning of plastic behaviour.
Since Hooke’s Law describes the behaviour of a material within the elastic limit, where stress is proportional to strain, we can write, $ F\; = -k \cdot x $ where $ F $ is the force, $ x $ is the length extended and $ k $ is the spring constant.
Therefore, Hooke's law essentially defines an elastic limit.
The correct answer is Option D.

Note
Both the yield point and elastic point are essentially the same point, but the difference is that they mark different limits. The elastic point marks the end of the elasticity region, while the yield point marks the beginning of the plasticity region.
Therefore it can obviously be said that the yield point is outside the elastic region. On application of an additional force $ F \to 0 $ at the elastic limit, the body starts yielding, displaying plastic behaviour.