Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
**Assertion A:** When a rod lying freely is heated, no thermal stress is developed in it.
**Reason R:** On heating, the length of the rod increases.
In the light of the above statements, choose the correct answer from the options given below:
Answer: (C) Both $A$ and $R$ are true but $R$ is NOT the correct explanation of $A$
Thermal stress appears only when the expansion of a rod is prevented. A freely lying rod is not restrained, so no stress develops: A is true.
A rod does lengthen on heating, so R is true. But the increase in length is not the reason for zero stress; the reason is that nothing opposes the expansion (a clamped rod also tends to lengthen and does develop stress). So R does not explain A.
Solution by Sreeraj P, M.Sc Physics