Tensile Stress in a Bar

Tensile stress is a type of stress that occurs when a material is subjected to pulling or stretching forces. It is defined as the force per unit area that is experienced by a material when it is stretched or pulled. In the case of a bar held between two rigid supports, tensile stress can occur under certain conditions. Let's explore these conditions in detail.

Heating and Cooling Effects on Tensile Stress

When a bar is heated or cooled, it undergoes thermal expansion or contraction, respectively. This means that its length changes due to the change in temperature. Now, let's consider the options provided in the question and analyze their effects on the bar:

a) Heated: When a bar is heated, it expands due to thermal expansion. As a result, its length increases, and it may exert a pulling force on the rigid supports. This pulling force leads to the development of tensile stress in the bar.

b) Cooled: When a bar is cooled, it contracts due to thermal contraction. As a result, its length decreases, and it may experience a compressive force from the rigid supports. However, in the context of the question, the bar is held between two rigid supports, which prevents it from contracting freely. Therefore, cooling alone does not lead to tensile stress in the bar.

c) Heated or Cooled: As discussed above, heating can cause tensile stress in the bar, while cooling does not have the same effect. Therefore, the statement that the bar will be subjected to tensile stress if it is heated or cooled is not entirely correct. Cooling alone does not lead to tensile stress.

d) Heated beyond the melting point: When a material is heated beyond its melting point, it undergoes a phase change from solid to liquid. In this case, the mechanical properties of the material change significantly, and it no longer behaves like a solid bar. Therefore, heating beyond the melting point does not cause tensile stress in the bar.

In Conclusion:

The correct answer to the question is option 'B' - the bar will be subjected to tensile stress if it is cooled. Cooling causes the bar to contract, but since it is held between two rigid supports, it is not able to freely contract. This results in the development of tensile stress in the bar.