Given below are two statements: one is labelled as Assertion A and...

Thermal stress is mechanical stress created by any change in temperature of a material. When the body is under some constraints, thermal stress is generated. But if it is lying freely, no thermal stress is developed.

The increase in length of a body with temperature is given by

$∆ L = L\alpha ∆ T$

So, with the increase in temperature, the length of the rod increases.

$A$ and $R$ are true but $R$ is not the correct explanation of $A$.